Feature to plot line using custom discrete points using GUI

[djmgit]

The terminal mode provides an option to plot a line using custom points.
For eg:
python plotit.py --line '[1,2,3,4],[4,10,5,9]'

The above option should also be present in GUI.
Presently the GUI provides an input field to provide a function by default. Instead of a default input field we can have an option (for example radio buttons) to select between plotting function, or plotting a custom line using custom points. If user selects the latter, then users will be provided a suitable way to provide the x and the y values and plot the function.