Department of Chemical Engineering ChE-101: Approaches to Chemical Engineering Problem Solving MATLAB Tutorial Vb

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1 Department of Chemical Engineering ChE-101: Approaches to Chemical Engineering Problem Solving MATLAB Tutorial Vb Making Plots with Matlab (last updated 5/29/05 by GGB) Objectives: These tutorials are designed to show the introductory elements for any of the topics discussed. In almost all cases there are other ways to accomplish the same objective, or higher level features that can be added to the commands below. Any text below appearing after the double prompt (>>) can be entered in the Command Window directly or in an m-file. For more details about plots read chapter 5 of the book. The following topics are covered in this tutorial; Introduction Using the plot Command Adding Labels, Text, or a Legend Saving Figures Printing Plots Customizing Axes Multiple data in a plot Linestyles, Markers, and Colors Plotting a Function Using fplot Using the pause Command Introduction The purpose of this handout is to cover the basics of creating, editing, and printing graphs. We will only cover 2_dimensional plots in this tutorial. There are considerably more commands that can be used to create more complicated 2-D and 3-D plots. They are covered in the Matlab manual. It is important that you review the rules for making quality plots that are given in H4. Plots in Matlab are created using vectors for the use of vectors and vectors operations you must review Tutorial III. The following commands will be covered in this tutorial: plot: creates a 2-D line plot axis: changes the aspect ratio of x and y axes xlabel: annotates the x-axis ylabel: annotates the y-axis title: puts a title in the plot prints: prints a hardcopy of the plot ===================================================================== Using the plot Command The plot command is used to create two dimensional plots. The simplest form of the command is 1

2 plot (x,y) Vector of independent Vector of dependent variable variable In order to show the plot command we need to create some arrays. Create the following m_file. MATLAB creates the plot by automatically scales the axes, marks the points and connects them with a straight line. It uses its default settings of line types, color, etc. This is what will be shown in the screen when you run the program We need to learn how to add labels and titles to the figure 2

3 Adding labels, title, and legend: There are many features in Matlab. Only the basics will be discussed here. For more details about options see Chapter 5 of the book. The commands xlabel and ylabel can be used to annotate the axis, the command title can be used to create a title of the plot, the command legend can be used to create a legend and the command text can be used to create a text anywhere in the plot. Examples of the syntaxes of these commands are given next: Check on different options This is the label for the axis This is the size for the font This is the color of the font xlabel( \fontname{select type, arial} title for the axis, Fontsize,e.g. 14, Color,e.g. r ) ylabel( \fontname{select type, arial} title for the axis, Fontsize,e.g. 14, Color,e.g. r ) title( \fontname{select type, e.g., arial} title for the graph, Fontsize,e.g. 14, Color,e.g. r ) Commands used for color are given next: Red: r Green: g Blue: b Black: k (this is the default). If you don t specify color Matlab will use black Magenta: m Yellow: y To add a plot legend to your graph, use the syntax; legend( string1, string2,,pos) Text for legend, the order of the text corresponds to the order the lines were created Pos is an optional number that specifies where in the figure the legend is placed. The options are given next. Pos = -1 places the legend outside the axes boundaries on the right side. Pos= 0 places the legend inside the axes boundaries in a location that interferes the least with the graphs Pos =1 places the legend at the upper-right corner of the plot (default) For more options see help legend in Matlab All the options must be written in the m.file after the plot command is used. Otherwise, they would not take effect in the program See the example given below for the use of the options described above. 3

4 This is how the plot will look like: 4

5 You can also manipulate the format of the figure once it is created by using the options given in the menu. You can also export your figure into a word document, power point, or WordPad. See instructions in the printing section of this tutorial. Play with some of the commands of the menu to learn what you can do: For example this is how you can insert a text: You can use this menu to modify some of the format of your plot Click on this box and write text where ever you whish This is how you can change the figure number: Select figure properties 5

6 Click on the title tag Type the text that you want Click OK tag This is what will be shown in your screen after the changes: This is really useful when your program generates different figures 6

7 Saving Figures You cannot save figures. You must save an m_file with all the commands used to create the figure. Get in the habit of using m_files to create figures because editing is much easier. ===================================================================== Printing Plots There are two ways to print plots.! The simplest way is print directly from the figure box.! The alternative is to copy the figure into WordPad. To do this; (a) In the figure box click on EDIT/COPY OPTIONS (b) Click on the Windows Metafile format (it is probably already on) This option 7

8 (c) Turn off the Match Figure Screen Size box (d) Turn on the White Background box (e) Click ON Click OK 8

9 (f) Click on EDIT/COPY FIGURE This option (g) Switch to a word processor document and paste where you want it. If you use a different word processing package you will have to try it with your package. There is a variation in there ability to import and adapt the colors. Customizing Axes There are a great number of axes commands given in detail in manual. The most common use will be; Changing the Default Axes Ranges axis([xmin xmax ymin ymax]) This lets you determine the range of the axes. Customizing the tick marks and ticklabels To set the tick marks to locations other than those selected by Matlab use the following command set(gca,'xtick',[a vector of points where you want the tick marks]) To set the tick labels to other than selected by Matlab use the following. These labels are considered to be text and are handled as such. This most commonly would be done to control the number of decimal points displayed. Each tick label entry in the list must have the same number of characters. If you would like some of the tick marks to be unlabeled, enter the label as the appropriate number of empty spaces. 9

10 set(gca,'xticklabel',['label1'; 'label2'; etc]) label1 and label2 would be something like 1.00, 2.00 etc. Using Log_Log and Semilog Axes We can also create log or semilog plots. In place of the word plot, use;! loglog(x,y)! semilogx(x,y)! semilogy(x,y) Add the following line to your program and see the changes in the figure This is what will be shown on the screen. Notice the new scale 10

11 Multiple Curves in a Plot Multiple plots can be put on the same graph. Vector of independent variable plot (x,y,x,z) Vector of dependent variable You can add as many vectors as needed Vector of dependent variable Vector of independent variable Example: Add the following array to the m_file and replace the plot command. z = [ ]; plot(x,y,x,z) This is the new data The legend was also changed 11

12 This is what will be shown on the screen: Now we need to learn how to change markers Linestyles, Markers, and Colors The Matlab Manual gives the available color, symbol types and linestyle options. If you don't specify Matlab starts with yellow and cycles through the list. The default linestyle is the solid line. The point symbols do not connect the data points with any line type. The general rule is if you have discrete data, use individual symbols (see Handout 3: Quality plots and tables). Using symbols with a linetype connecting them is acceptable if needed to help the reader follow the data. The syntax for changing the default color or linestyle settings is plot (x,y, line specifiers ) Vector of independent Vector of dependent variable variable See some of the options below. Look at the Matlab help for many more Some line specifiers are given below Solid (default): - Dashed: -- Dotted: : Dash-dot: -. 12

13 All the above options can be combined with the color code given on p. 3 of this tutorial. Some marker types are given below: Plus sign: + Circle: o Asterisk: * Point:. Square: s Diamond: d Five-pointed star: p Six-pointed star: h Some examples are given below: plot(x,y, r ) A red line will connect the points plot(x,y, --y ) A yellow dashed line connects the points plot(x,y, * ) The points are marked with * (no line between the points) plot(x,y, g:d ) A green dotted line connects the points that are marked with diamond markers Modify your m.file to print the data with different markers: See the changes to have different markers and colors. Look at H3 for rules 13

14 This is what will be shown in the screen The markers make it easier to distinguish when you print in black and white. Always use different markers do not rely on colors to distinguish. See H3 The experimental data is not joined Data from equations is joined Plotting a Function Using the fplot Command We can also plot a function of one variable between specified limits without creating a data set. When you have a function you should use fplot so that it appears as a smooth curve, not as discrete points. The syntax is fplot('function',[xmin xmax]). Where function is the function to be plotted and xmin and xmax are the independent variable limits. This option is explained with details in section 5.2 of the book. 2 Example: Create a function file that will perform the following operation. z sin ( x) is the independent variable and z is the dependent variable. =, where x 14

15 Create the following main file that will pass the input information to the function and will plot the results of the function. Remember that to make a plot you need several data points, therefore, your independent variable should be a vector, that way you will have multiple values for the dependent variable. This way you plot the function directly This is what will be shown in the command window: 15

16 However, the above option does not allow you to control the plot options to its best. My recommended choice (this option is explained with details in section of the book) is to use the function to create your own dependent variable vector and then plot using the plot command instead. See the example below that shows how to do (the changes are done only in the main file, this is one way to do it). This option uses the for loop without modifying the function file This option uses element by element operation and both the function file and m.file need to be modified as shown: Function file: Notice the element by element operation used here 16

17 This is the m.file using the new function Because element by element operation is used in the function file a for loop is not needed in the main program If you run the program this is what will be shown in the screen (either option): The points are not smooth because more points are needed 17

18 If you want to add more points modify the main program as shown (you don t need to modify the function file, use the function files that uses the element by element operator: Need to create more points in the x vector This is how your plot will look like now, because you have more points in the x vector: 18

19 Using the Pause Command If you are creating a plot as part of a larger Matlab program, you may want the program to create the plot and then wait for you to examine it before proceeding. This is done using the pause command after the plot command. After creating the figure, Matlab will wait until any key is hit before continuing. ===================================================================== Congratulations! You know now how to make plots in Matlab. Need to follow the instructions given in H3:Quality Plots in addition to what you learned from this tutorial Matlab has many other options, you can learn them using the help command for plots 19