Version of OpenTTD

from 20210114-master (applies to change #8557)

Expected result

1. The auxiliary grid helps in reading the values, but it does not impose itself and does not tire your eyesight.
2. The 0 value line is clearly visible and stands out from the other lines.
   

Actual result

1. The main auxiliary grid is too light and creates too much contrast against the background, which reduces the readability of the graphs. This contrast is also tiring for the eyesight. In the case of a light background, the difference in shades between the background and the lines is 1. Here, the difference is as much as 5.
2. The important line of value 0, which should be clearer than the others, is black and therefore almost invisible.
   

Solutions

Below in the graphic I have marked which colors are used for the light background of the chart, those for the current one and those that I used in my graphics.

1. The auxiliary grid works best when its contrast against the background is 1.
2. For the color of the 0 value line, the contrast against the background should also be similar to that on a light background and amount to 5-7.
3. On my graphics I used an additional shade to mark the line separating the game years - they are 1 shade lighter than the auxiliary grid