Remove a line in xkcd_plot.dart that is no longer used

I forgot to remove it in the last commit
Display warning message right below the graph
Add 'for' attribute to labels
A new layout for input elements
Use fieldset instead of div to group input fields
Show indexes of equations

Equation 1, Equation 2, etc.
New style for buttons
Remove no longer used style from css
Add some style to the UI
New colour for buttons