improve visualisation of relation analysis chart

[dameiers]

currently the axes of the relation analysis charts are dynamically adopted to the present data set. this can lead to messy visualisation, especially if only two worldstates are selected.

One idea to fix this is to use two special points that mark the lower and upper bound of the axes. the x / y values of these points need to be editable.
Those special point can be renderen in a seperate way like other data points.