PlotHistogram: bars are drawn based on the position of zero (#828)

imgui.cpp

@@ -7163,7 +7163,8 @@ void ImGui::PlotEx(ImGuiPlotType plot_type, const char* label, float (*values_ge
 
         float v0 = values_getter(data, (0 + values_offset) % values_count);
         float t0 = 0.0f;
-        ImVec2 tp0 = ImVec2( t0, 1.0f - ImSaturate((v0 - scale_min) / (scale_max - scale_min)) );    // Point in the normalized space of our target rectangle
+        ImVec2 tp0 = ImVec2( t0, 1.0f - ImSaturate((v0 - scale_min) / (scale_max - scale_min)) );                       // Point in the normalized space of our target rectangle
+        float histogram_zero_line_t = (scale_min * scale_max < 0.0f) ? (-scale_min / (scale_max - scale_min)) : (scale_min < 0.0f ? 0.0f : 1.0f);   // Where does the zero line stands
 
         const ImU32 col_base = GetColorU32((plot_type == ImGuiPlotType_Lines) ? ImGuiCol_PlotLines : ImGuiCol_PlotHistogram);
         const ImU32 col_hovered = GetColorU32((plot_type == ImGuiPlotType_Lines) ? ImGuiCol_PlotLinesHovered : ImGuiCol_PlotHistogramHovered);
@@ -7178,7 +7179,7 @@ void ImGui::PlotEx(ImGuiPlotType plot_type, const char* label, float (*values_ge
 
             // NB: Draw calls are merged together by the DrawList system. Still, we should render our batch are lower level to save a bit of CPU.
             ImVec2 pos0 = ImLerp(inner_bb.Min, inner_bb.Max, tp0);
-            ImVec2 pos1 = ImLerp(inner_bb.Min, inner_bb.Max, (plot_type == ImGuiPlotType_Lines) ? tp1 : ImVec2(tp1.x, 1.0f));
+            ImVec2 pos1 = ImLerp(inner_bb.Min, inner_bb.Max, (plot_type == ImGuiPlotType_Lines) ? tp1 : ImVec2(tp1.x, histogram_zero_line_t));
             if (plot_type == ImGuiPlotType_Lines)
             {
                 window->DrawList->AddLine(pos0, pos1, v_hovered == v1_idx ? col_hovered : col_base);