[cartesian] improve gridlines z-index

[markov00]

If different grays are used on axis line and grid lines, there can be overlaps that don't look nice as shown in the following example:

There could be a few possible solutions to this:

• allow using only one color for grid/axis lines
• render always the axis after rendering the grid but this will not solve the problem if the grid lines have different shades.

When different shades are used, then probably we need to sort the rendering order by color or by importance and render first light colors and then darker colors or more important elements ontop of less important elements.

Here is a demo PR to prove the point #2122