[Bug] PlotBars baseline=0 breaks on log-scaled axes

[Gidid56]

Bug: PlotBars baseline=0 breaks on log-scaled axes

When using ImPlot::PlotBars(...) with a log-scaled value axis (ImPlotScale_Log10), the bars are drawn with a baseline of 0. Since 0 is invalid on logarithmic axes, this can cause bar geometry to explode and appear to draw outside the plot boundaries / clip rect.

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Minimal Repro

static double values[] = { 1, 10, 100, 1000 };

if (ImPlot::BeginPlot("Bars + Log Axis")) {
    ImPlot::SetupAxisScale(ImAxis_Y1, ImPlotScale_Log10);
    ImPlot::SetupAxisLimits(ImAxis_Y1, 1e-1, 1e9);
    ImPlot::PlotBars("bars", values, 4, 0.8);
    ImPlot::EndPlot();
}

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Observed

Bars may extend far beyond the plot area (or appear to draw outside plot boundaries) when the value axis uses ImPlotScale_Log10.

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Expected

Bars should start at a log-safe baseline (e.g. the current axis minimum), not at 0, when the value axis is logarithmic.

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Proposed Fix (small, localized)

Clamp the baseline used by PlotBars from 0 to a log-safe value when the value axis is log-scaled:

• If the relevant axis scale is ImPlotScale_Log10, use axis.Range.Min if > 0, otherwise fall back to a small epsilon (e.g. 1e-12).
• Keep the baseline behavior unchanged for linear axes.

This preserves existing rendering/legend/toggle behavior and only replaces an invalid baseline on log scales.

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Patch Summary (implot_items.cpp)

The issue comes from getter2 always using IndexerConst(0) as the baseline. The fix is to replace that 0 with a log-safe baseline when the value axis is log-scaled.

Example (conceptually):

// before (vertical bars)
GetterXY<IndexerLin,IndexerConst> getter2(IndexerLin(1.0,shift), IndexerConst(0), count);

// after (vertical bars)
const double base = GetLogSafeBaseline(ImAxis_Y1, 0.0);
GetterXY<IndexerLin,IndexerConst> getter2(IndexerLin(1.0,shift), IndexerConst(base), count);

Same idea applies to:

• horizontal bars (value axis is X1)
• PlotBars(xs, ys, ...)
• PlotBarsG(...)

I have a working patch applied locally and can provide the full diff if desired.

[brenocq]

I modified the Demo_BarPlots to try to reproduce your bug, but the bug didn't appear

void Demo_BarPlots() {
    static ImS8  data[10] = {1,2,3,4,5,6,7,8,9,10};
    if (ImPlot::BeginPlot("Bar Plot")) {
        ImPlot::PlotBars("Vertical",data,10,0.7,1);
        ImPlot::PlotBars("Horizontal",data,10,0.4,1,{ImPlotProp_Flags, ImPlotBarsFlags_Horizontal});
        ImPlot::EndPlot();
    }
+
+    static double values[] = { 1, 10, 100, 1000 };
+    if (ImPlot::BeginPlot("Bars + Log Axis")) {
+        ImPlot::SetupAxisScale(ImAxis_Y1, ImPlotScale_Log10);
+        ImPlot::SetupAxisLimits(ImAxis_Y1, 1e-1, 1e9);
+        ImPlot::PlotBars("bars", values, 4, 0.8);
+        ImPlot::EndPlot();
+    }
}

Can you double check if you still have this issue with the latest master branch?

[Gidid56]

Here is my configuration:

Windows 11 25H2 (Build 26200.7840)
Magnum SDL2Application + ImGuiIntegration::Context
Dear ImGui (IMGUI_VERSION_NUM 19259)
ImPlot (IMPLOT_VERSION_NUM 1801)

GL_RENDERER: Intel(R) UHD Graphics
GL_VERSION : 4.6.0 - Build 27.20.100.9168

The minimal example reproduces the issue on my machine:

I’m fairly sure this used to render correctly on my setup a few years ago; however I can’t identify the exact ImPlot/ImGui/Magnum/driver version where it changed.

Local Patch

Clamping the baseline to a positive value when the value axis is logarithmic resolves the issue locally:

// Helper: returns a baseline that is safe for log axes (never <= 0 on log)
static inline double GetLogSafeBaseline(ImAxis axis, double requested_baseline) {
    // On linear axes, keep the requested baseline (usually 0)
    // On log axes, clamp baseline to axis-range minimum (if valid) or epsilon.
    if (requested_baseline > 0.0)
        return requested_baseline;

    if (GImPlot == nullptr)
        return 1e-12;

    ImPlotContext& gp = *GImPlot;
    ImPlotPlot* plot = gp.CurrentPlot;
    if (plot == nullptr)
        return 1e-12;

    const ImPlotAxis& ax = plot->Axes[axis];

    if (ax.Scale == ImPlotScale_Log10) {
        const double axis_min = ax.Range.Min;
        return (axis_min > 0.0) ? axis_min : 1e-12;
    }

    return requested_baseline;
}

template <typename T>
void PlotBars(const char* label_id, const T* values, int count, double bar_size, double shift, const ImPlotSpec& spec) {
    if (ImHasFlag(spec.Flags, ImPlotBarsFlags_Horizontal)) {
        const double base = GetLogSafeBaseline(ImAxis_X1, 0.0);
        GetterXY<IndexerIdx<T>, IndexerLin> getter1(IndexerIdx<T>(values, count, spec.Offset, Stride<T>(spec)), IndexerLin(1.0, shift), count);
        GetterXY<IndexerConst, IndexerLin>  getter2(IndexerConst(base), IndexerLin(1.0, shift), count);
        PlotBarsHEx(label_id, getter1, getter2, bar_size, spec);
    }
    else {
        const double base = GetLogSafeBaseline(ImAxis_Y1, 0.0);
        GetterXY<IndexerLin, IndexerIdx<T>> getter1(IndexerLin(1.0, shift), IndexerIdx<T>(values, count, spec.Offset, Stride<T>(spec)), count);
        GetterXY<IndexerLin, IndexerConst>  getter2(IndexerLin(1.0, shift), IndexerConst(base), count);
        PlotBarsVEx(label_id, getter1, getter2, bar_size, spec);
    }
}

The same adjustment was applied to the other PlotBars overloads.