Line graph shading should not be limited to just vertical shading

[TCL735]

Line graph shading is very primitive and needs improvement. The "shadeBelow" option is very literal. It does not care that a line graph can be flipped sideways (time as the y-axis, value as the x-axis).

Proposal: "shadeBelow" should be smarter: allow the user to specify the shaded dimension, and have sensible defaults that follows the time axis by shading appropriately.

Here is an example of incorrect shading when time axis is flipped with the value axis:

The correct shading would be shading that goes from the vertical axis' border on the left to the line points, ie left-to-right shading.

[TCL735]

Related to #668