Add corresponding GUI option for plotting discrete points

[djmgit]

The terminal mode provides an option to plot discrete points.
For eg:
python plotit.py -f x --points '[2,3,4,5]'
The GUI should also provide the above option.
A possible way could be adding an button (something like plot points), clicking on which which open a sub window which will have option to provide a list of points. After entering the list user will press done (enter should also work) and the function will be plotted.