Have separate variables for top, left, right and bottom margins in Axes

[v0dro]

Currently the margins (top/bottom and left/right whitespace after which the X & Y axis begin) are defined by two variables @x_axis_margin and @y_axis_margin. As a result we cannot customize 4 sides individually.

Change these to @top_margin, @left_margin, @bottom_marginand@right_margin`. Changes should reflect in the other parts of the code too.