public int[] getPaletteHistogram() {
        if (!paletteHistogramSet) {
            throw new IllegalStateException(PropertyUtil.getString("PNGEncodeParam14"));
        }
        return paletteHistogram;
    }
Returns the palette histogram to be stored with this image.

<p> If the histogram has not previously been set, or has been
unset, an <code>IllegalStateException</code> will be thrown.

@throws IllegalStateException if the histogram is not set.
plugins/svg-plugin/batik-codec-fix/src/main/java/org/apache/batik/ext/awt/image/codec/png/PNGEncodeParam.java