OK, I can call ShowSeriesLegend(false) (what about having NoSeriesLegend here?) to remove the spline legend line, but I would also wish to have green line with that red diamond. I think the fun is spoiled by
if (serie.opacity > 0 && serie.thickness > 0 && serie.seriesPlot)
serie.seriesPlot in Legent.cpp. I guess we would need something like (serie.seriesPlot || serie.legendLine) there...