The lines connecting the tick marks of the axis are called splines.
There are four of them: top/bottom/left/right
So far, they have margined our graphs.
They can be placed at arbitrary positions.
We want to place them in the middle now.
To achieve this, we will discard the top and right ones by setting their color to none and we will move the bottom and left ones to coordinate 0 in data space coordinates.
top spine
left spine
bottom spine
right spine