Transparent Areas/Stacks are being overlayed, resulting in odd lines showing

[thurban]

Describe the bug

To Reproduce
Charts with transparent colors show odd lines when using Area/Stack. The different areas are being overlayed on each other which cause the part where they overlap to be with a stronger color.

Expected behavior
The different areas should not overlap each other.

Screenshots
Chart showing Issue ( 0.2 value )

Chart showing fixed ( 0 value )

Desktop (please complete the following information):
rrdtool 1.6.0, 1.7.0, 1.7.1, 1.8.0

Additional context

The overlap is caused by the "0.2" in the following code from rrd_graph.c:

if (im->slopemode == 0) {
                            backY[++idxI] = ybase - 0.2;
                            backX[idxI] = ii + im->xorigin - 1;
                            foreY[idxI] = ytop + 0.2;
                            foreX[idxI] = ii + im->xorigin - 1;
                        }
                        backY[++idxI] = ybase - 0.2;
                        backX[idxI] = ii + im->xorigin;
                        foreY[idxI] = ytop + 0.2;
                        foreX[idxI] = ii + im->xorigin;

Then setting the 0.2 to "0" ( not adding/removing anything from ytop/ybase) the graph show up just fine

Non-Transparent Areas are not affected ( or better: the issue is not being seen due to the non-transparent color).