* Plotting Experimental Data
*** Outline
***** Introduction
******* Why do we want to do that?
******* Inputting data
********* Lists
********* Files
******* Arsenal Required
********* Lists
*********** initializing
*********** appending to lists
********* for loops
*********** iterating over a list
********* basic plotting
***** Inputting data as lists
******* Input the dependent and independent variables in two lists
******* Plot the points
***** Plotting from files
******* Look at the file
******* Read the file and get data into variables
******* Do any massaging required
******* Plot the points

*** Script

This video will teach you how to plot experimental data, with two
variables. 

In general, we don't plot (analytical) functions. We often have
experimental data points, that we wish to plot. We shall look at
inputting this data and plotting it. 

The data could be input (or entered) in two formats. For smaller data
sets we could use lists to input the data and use plain text files for
(somewhat?) larger ones. (Binary files?)

[[[Before starting with this video, you should be comfortable with
  - Lists
    - initializing them
    - appending elements to lists
  - for command
    - iterating over a list
  - split command
  - plot command
    - plotting two variables
]]]

Let's begin with inputting the data as lists and plotting it. 

x = [0, 1, 2.1, 3.1, 4.2, 5.2]
y = [0, 0.8, 0.9, 0, -0.9, -0.8]
Now we have two lists x and y, with the values that we wish to plot. 

We say:
plot (x, y, 'o')

and there, we have our plot!

[We close the plot window. ]

Now, that we know how to plot data which is in lists, we will look at
plotting data which is in a text file. Essentially, we read the data
from the file and massage it into lists again. Then we can easily
plot it, as we already did. 

As an example we will use the data collected from a simple pendulum
experiment. We have the data of, the length of pendulum vs. the time
period of the pendulum in the file pendulum.txt

In []: cat pendulum.txt (windows?)

The cat command, shows the contents of the file. 

The first column is the length of the pendulum and the second column
is the time. We read the file line-by-line, collect the data into
lists and plot them. 

We begin with initializing three empty lists for length, time-period
and square of the time-period. 

l = []
t = []
tsq = []

Now we open the file and read it line by line. 
for line in open('pendulum.txt'):

We split each line at the space 
    point = line.split()

Then we append the length and time values to the corresponding
lists. Note that they are converted from strings to floats, before
appending to the lists
    l.append(float(point[0])
    t.append(float(point[1])
We also calculate the squares of the time-period and append to the end
of the tsq list. 
    tsq.append(t[-1]*t[-1])
As you might be aware, t[-1] gives the last element of the list t. 

Now the lists l, t have the required data. We can simply plot them, as
we did already. 

plot(l, t, 'o')

Enjoy!

*** Notes
    - Also put in code snippets?