Add PlotShadedClip, fills region only if the second line is higher than the first one.

[WildRackoon]

PlotShaded only supports a single color for the overlap region, It could use a segregation based on a quantity being higher than another.

Here is a Simple example, used to visually assert wether a stock market's limit orders are appropriate for transactions to be done:

This is achieved by two PlotShadedClip calls, one from ys1 to ys2 and a second from ys2 to ys1;

//ClipShades
ImPlot::SetNextFillStyle(ImVec4(1.0f,0.0f,0.0f,0.25f));
ImPlot::PlotShadedClip  ("Market Bad" , &history_price_buy.Data[0].x , &history_price_buy.Data[0].y , &history_price_sell.Data[0].y, history_price_buy.Data.size() , history_price_buy.Offset , 2*sizeof(float));
ImPlot::SetNextFillStyle(ImVec4(0.0f,1.0f,0.0f,0.25f));
ImPlot::PlotShadedClip  ("Market Good", &history_price_sell.Data[0].x, &history_price_sell.Data[0].y, &history_price_buy.Data[0].y , history_price_sell.Data.size(), history_price_sell.Offset, 2*sizeof(float));

The Implementation is very basic, it comes down to 2 modified lines in comparison with PlotShaded, upper vertices coords are squeezed down if relative height conditions are not met.

DrawList._IdxWritePtr[0] = (ImDrawIdx)(DrawList._VtxCurrentIdx);
DrawList._IdxWritePtr[1] = (ImDrawIdx)(DrawList._VtxCurrentIdx + 1 + intersect);
DrawList._IdxWritePtr[2] = (ImDrawIdx)(DrawList._VtxCurrentIdx + 0 + 3 * (P11.y >= P12.y) );
DrawList._IdxWritePtr[3] = (ImDrawIdx)(DrawList._VtxCurrentIdx + 1);
DrawList._IdxWritePtr[4] = (ImDrawIdx)(DrawList._VtxCurrentIdx + 3 - intersect);
DrawList._IdxWritePtr[5] = (ImDrawIdx)(DrawList._VtxCurrentIdx + 1 + 3 * (P21.y >= P22.y) );
DrawList._IdxWritePtr += 6;
DrawList._VtxCurrentIdx += 5;

This PR only a baseline for a better implementation of this kind of feature, it also only does ShadeClipping between two series, not with a single serie and a ref, simply because I didn't know how to elegantly handle the choice between a shaded region above or under the reference line. (switching positional arguments doesn't seem very clean to me, but its work).

EDIT: I mean extending this to lines, scatter etc... and have dual/multi color thresholds would need significant changes.

Looking forward to your suggestions.

[epezent]

@WildRackoon , thanks for the PR. This looks useful! I'll try to take a look soon (currently swamped with school).

[WildRackoon]

Rebased this to latest code changes just in case anyone needs this
Added Demo example
But still no idea to reduce code duplication in an intuitive manner / handle