* Script

*Hello and welcome to the tutorial on Basic Plotting using Python. This is the first tutorial in a series of tutorials on Python for Scientific Computing. This tutorial is created by the FOSSEE team, IIT Bombay .  

*The intended audience for this tutorial are Engineering, mathematics and science teachers and students

*The goals are to

help one use Python as a basic plotting tool.

*In this tutorial, we will cover the basics of the Plotting features available in Python. 

For this we shall use  Ipython and pylab. 

Ipython is An Enhanced Interactive Python interpreter. It provides additional features like tab completion,easier access to help , and many other useful features which are not present in the vanilla Python interpreter.

Pylab is python library which provides plotting functionality. 

I am assuming that you have both Ipython and Pylab installed on your system .

*On your terminal type in the command Ipython -pylab

$ ipython -pylab

press RETURN

We will first start with the absolute basic, that is how to print hello world

In []: print 'hello world'

Voila we have got hello world as the output 

To exit ipython press Ctrl-D.

*Now we will get back to plotting.

type again :

$ ipython -pylab

press RETURN

First, we will create a sequence of equally spaced points starting from 0 to 2*pi , we will use function linspace for that

Type:

In []: x = lins<Tab> This is an Ipython feature that will auto-suggest the word

oops  I made a mistake . As you can see I made the mistake of not writing command correctly

and Ipython changed the prompt . To get the old prompt back type crtl-c

In []: x = linspace(0, 2*pi, 50)

In []: plot(x, sin(x))

***

As you can see a plot has appeared on the screen. 

***

The plot has the default color and line properties. 

*To know more about any function,  example for the 'linspace' function you can type ? after it .

In []: linspace?

*As you can see linspace can take three parameters start, stop, and num and returns num evenly space points . You can scroll through the help to know more about the function

In this case we have used two commands 

*Now that we have a basic plot, we can label and title the plot. 

In []: xla<TAB>bel('x') will add a label to the x-axis. Note that 'x' is enclosed in quotes. 

Similarly

In []: ylabel('sin(x)') adds a label to the y-axis.

To add a title to plot we simply use 

In []: tit<TAB>le('Sinusoid').

Add a legend to the plot by typing 

In []: legend(['sin(x)'])

We have just typed legend command can we reuse it . Yes 

To go to previous command, we can use 'UP Arrow key' and 'DOWN' will take us (in reverse order)/back.

We can modify previous command to specify the location of the legend, by passing an additional argument to the function. 

#Ask madhu how to describe the feature here.

In []: legend(['sin(x)'], loc = 'center')

Note that once 

other positions which can be tried are

'best' 

'right'

In this case, let's add a comment at the point of origin. 

In []: annotate('origin', xy=(0, 0))

The first argument is the comment string and second one is the position for it. 

Ok, what do I do with all this effort . I obviously have to save it . 

We save the plot by the function savefig

In []: savefig('sin.png') saves the figure with the name 'sin.png' in the current directory. 

#other supported formats are: eps, ps, pdf etc.

When we use plot again by default plots get overlaid.

In []: plot(x, cos(x))

we update Y axis label 

In []: ylabel('f(x)')

Now in these situations with overlaid graphs, legend becomes absolutely essential. To add multiple legends, we pass the strings within quotes separated by commas and enclosed within square brackets as shown.

In []: legend( [ 'sin(x)' , 'cos(x)'] )

In []: clf()

clears the plot area and starts afresh.

*In case we want to create multiple plots rather than overlaid plots, we use 'figure' function.

The figure command is used to open a plain figure window without any plot.

In []: figure(1)

We will use plot() plot again to plot a sin curve on figure(1)

In []: plot(x, sin(x))

to creates a new plain figure window without any plot type: 

In []: figure(2)

figure() is also used to shift the focus between multiple windows. 

Any command issued henceforth applies to this window only so typing

In []: plot(x, cos(x))

plots cos curve on second window now.

The previous plot window remains unchanged to these commands.

calling function figure using argument 1 shifts the focus back to figure(1).

In []: figure(1)

title() sets the title of figure(1) 

In []: title('sin(x)')

Now we save the plot of figure(1) by typing 

In []: savefig('sine.png')

close() closes figure(1). Now there is just one figure that is open and hence 

the focus is automatically shifted to figure(2).

In []: close()

close() now closes the figure(2).

In []: close()

The plot command can take the  parameters such as 'g' which generates the plot in green color. 

Passing the linewidth=2 option to plot, generates the plot with linewidth of two units.

Use Up arrow to get to the previous commands 

In []: plot(x, sin(x), 'g', linewidth=2) and add arguments

In []: clf()

In order to plot points you may pass '.' as a parameter to plot

In []: plot(x, sin(x), '.')

In []: clf()

You may look at more options related to colors and type of lines using plot?(question mark)

In []: clf()

and finally to close the plot

In []: close()

In this tutorial You learned 

Ipython Features

Tab for autocompletion.

function? to look at documention

q to quit documentation.

escapping from ... prompt using Ctrl -C

quitting ipython using Ctrl-D

The functions and commands you learnt were :

print for outping things

linspace to create equally space points

plot to get a plot

xlabel to label x-axis of plot

ylabel to label y-axis of plot

title to title a plot

legend for placing  appropriate legend

annotating a plot

saving a plot

multiple plots using figure

clearing plots using clf()

and last closing plot using close()

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This brings us to the end of this tutorial.  Thank you for attending this on Python for Scientific Computing .Hope you enjoyed it and found it useful.

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