Gnuplot/C2/Plotting-functions/English

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Visual Cue Narration



Slide Number 1

Title Slide Plotting functions

Welcome to the tutorial on Plotting functions in gnuplot.
Slide Number 2

Learning Objectives

In this tutorial we will learn,
  • Plot and replot commands for 2D plotting
  • Plot three functions: sin x, cos x and sin x plus cos x
  • Draw grid and set limits
  • Zoom in and zoom out of a plot


Slide Number 3

Learning Objectives

* Add axis labels and graph title
  • Change font style and size
  • Add Greek symbols
  • Set axis tick marks and
  • Save the data points to a file.


Slide Number 3

System and Software Requirement


To record this tutorial, I am using


  • Debian Linux 9.3
  • gnuplot 5.2.6
  • gedit text editor 3.22
Slide Number 4

Pre-requisites

To follow this tutorial,
  • Learners must be familiar with high school level Mathematics.
  • For pre-requisite gnuplot tutorials, please visit this site.
Press Ctrl Alt T Open a terminal by pressing Ctrl, Alt and T keys simultaneously.
Type cd Desktop and press Enter A terminal opens.

Change directory to Desktop as seen.

>gnuplot

Press Enter

In the terminal, type gnuplot and press Enter.
Hover mouse near gnuplot prompt The prompt changes to gnuplot prompt.

To execute a command in the terminal, press Enter after typing it. I will not mention this separately.

Press Ctrl shift K I will clear the terminal.
Type plot sin (x)

Press Enter

Now type plot space sin (x)

Note that angle x is within parentheses.

Point to graphical window A graphical window with plot of sin(x) versus x appears on the screen.
Hover mouse on graph, axes. The limit of the Y axis is from minus 1 to plus 1.

And the X axis is from -10 to 10.

Type set grid

Press Enter

Come back to the terminal and type the command set space grid.
Type replot

Press Enter

Then type replot to see the grid in the graph.
Hold Ctrl and zoom in with mouse scroll Hold down the Control key and move the mouse scroll wheel to zoom in and zoom out.
Point to sin x and x Observe that sin x is zero when x is zero.
Show zoom in and out. Let us reset the limits of the axes as before.
Type reset

Press Enter

To reset, type the command reset in the terminal, followed by replot.
Type replot

Press Enter

Changes in the graph become visible only after executing the replot command.
Point to the legend Notice that the legend is on the top right side in the graph.

It is not clear and intersects the graph.

Point mouse on the legend. Let's increase y axis range, so that the graph line does not intersect the legend.
Type set yrange [-2:2]

Press Enter

Type set space yrange space -2 colon 2 within square brackets.
Type set xrange [-20:20]

Press Enter

I will also change x axis range.

Enter command, set space xrange space -20 colon 20 within square brackets.

Type replot

Press Enter

Type replot to see the updated graph.
Now the changed graph limits are visible.
Point to the legend. The legend has moved up and does not intersect the data line anymore.
Type set key top left

Press Enter

Type set space key space top space left to move the legend to the top left side.
Type replot

Press Enter

Do not forget to replot to see the result.
Hover mouse next to axis font. Next, let's make the axes fonts larger.

I want to set them at size 14 and in Verdana style.

Type set tics font “Verdana, 14”

Press Enter

Type this command as shown here, to change the font size and style.

Replot to see the result.

Hover mouse next to axis font. Notice the larger font size in Verdana style.
Type set xlabel 'x (angle)' font “Verdana, 14”

Press Enter

Let's set x axis label as x angle.

Type the command as shown here. Here we are specifying the label, font style and size in a single command.

Type set ylabel 'sin (x)' font “Verdana, 14”

Press Enter

To set y axis label as sin x, type the command, as shown here.
Only narration We are using the same font size and style for y axis too.
Type replot

Press Enter

Type replot and press Enter.
Only narration Now, let’s see how to change the font in the legend.
Type set key font ",16"

Press Enter

Type

set space key space font space within double quotes comma 16 This command sets the legend font size to 16.

Press Ctrl+Alt+K I will clear the terminal.
Type help fonts

Press Enter

To access help documentation on fonts, type help space fonts.
Point mouse to cairo, gd and postscript Help command returns with 3 subtopics cairo, gd and postscript.
Type cairo

Press Enter

I will type cairo to see the details and available options.
Only narration Similarly you may choose to use help for any command when using gnuplot.
Mouse in Help section You may pause the video and read the details given in the help section.
Press Ctrl C Once done, press Control and C keys to exit the help section.
Ctrl L in Ubuntu

Ctrl Shift K in Debian

I will clear the screen.

I wish to add a title to my graph.

Type set title 'My first gnuplot'

Press Enter

To add a title, type set title command, as shown here.

Followed by the title of the graph in single quotes.

Type set title font "verdana, 18"

Press Enter

To set the title font, type the command as shown here.
Point to x axis In the sinx versus x plot, dimensions of x axis is angle.

Hence we may want to label X axis in terms of angle, π.

In the terminal, type,

set xtics ('-4{/Symbol p}' -4*pi,'-3{/Symbol p}' -3*pi,'-2{/Symbol p}' -2*pi, '-{/Symbol p}' -pi, 0, '{/Symbol p}' pi,'2{/Symbol p}' 2*pi, '3{/Symbol p}' 3*pi, '4{/Symbol p}' 4*pi,) >enter

To change X ticks to angle, type the following command as shown on the screen.

Here we are using the Greek alphabet pi and setting its position.

Point mouse to ('2{/Symbol p}' and then to 2*pi This command places, Greek symbol pi, at the value of 3.14.

Label 2π is at position 6.28 on the X axis.

Point to 2π and 4π. Multiples of pi are placed at their respective positions.
Type help set xtics

Press Enter

For more help on the formats of X tics, type the command help space set space xtics.
Press Ctrl C Press Ctrl and C keys to exit the help section.
I will clear the terminal.
Only narration It is also possible to add multiple functions on the same graph.
Type plot sin (x), cos (x)

Press Enter

To plot sin x and cos x on the same graph, separate the functions by a comma.

Type plot space sin x comma cos x Make sure that the angle x, is within parentheses .

Point to the graph window Notice that the two functions are plotted together now.
Only narration Next, we will add sin (x) plus cos (x) function to the same graph.
Type plot sin(x), cos(x), sin(x) + cos(x)

Press Enter

For this, type plot space sin(x) comma cos(x) comma sin(x) plus cos(x) .
Point to the graph window Notice that all 3 functions are plotted together on the same graph now.

Next, let’s learn how to get sin(x) data points into a file.

Only narration To get an ASCII column format output, set table command can be used.
Type set space table

Press Enter

First type set space table.
Enter command plot sin(x) and press Enter Then type plot space sin(x).
Point to the screen An x y column data for sin(x) is generated on the screen.
Highlight first column

Highlight second column

The first column is x data and the second column is y data.

They are the points of sin x versus x graph.

Press Ctrl+Alt+K I will clear the terminal.

We can save the output as a text file.

Type set table "table-sinx.txt".

Press Enter

For this, type set space table space within double quotes sinx dot txt


Here sinx dot txt is the output filename.

Type plot sin(x)

Press Enter


Then, type plot space sin(x).

The output is saved to the file sin x dot txt. The data is not seen on the screen,

Show screenshot of Desktop showing the file sinx.txt. The file is stored in the directory from where we opened gnuplot in the terminal.
Type unset table

Press Enter

Type unset space table to exit the tabular output mode.
Enter q Then type quit or q to quit gnuplot.
Slide Number 6

Summary

Let’s summarize.

In this tutorial, we plotted 2D graphs of functions.

We also

  • Added grid and set limits
  • Zoomed in and zoomed out of the graph
  • Added axis labels and graph title
  • Changed font style and size and
  • Saved the data points to a file


Slide Number 7

Assignment

As an assignment, plot the following functions:
  • tan x
  • x squared and
  • x minus three squared
Completed assignment Your completed assignment should look like this.
Slide Number 8

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Please download and watch it.

Slide Number 9

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Slide Number 10

Forum for specific questions:

Post your timed queries in the forum.
Slide Number 11

AcknowledgemenT

Spoken Tutorial Project is funded by NMEICT, MHRD, Government of India.
This is Rani from IIT, Bombay. Thank you for joining.

Contributors and Content Editors

Madhurig, Ranipv076