Add TestForIssue481 demo - multiple histogram x-axis alignment

[timmolter]

Issue #481 reported that combining two Histogram instances with non-overlapping data ranges onto the same CategoryChart produces an incorrect x-axis. This is not a bug in the library - it is a usage pattern that requires explicit configuration.

Root cause: Histogram(data, numBins) auto-detects its own min/max from its data. When two histograms have different ranges they generate disjoint bin-center lists, and CategoryChart (which uses x-axis values as categorical labels) cannot reconcile them automatically.

Fix / correct usage: supply the same shared global [min, max] to every Histogram using the explicit-range constructor:

double globalMin = -2.5;
double globalMax = 2.5;
int numBins = 5;

Histogram h1 = new Histogram(Arrays.asList(0, 1, 2),   numBins, globalMin, globalMax);
Histogram h2 = new Histogram(Arrays.asList(-2, -1, 0), numBins, globalMin, globalMax);

chart.addSeries("series A", h1.getxAxisData(), h1.getyAxisData());
chart.addSeries("series B", h2.getxAxisData(), h2.getyAxisData());

Note: to get bin centers at exact integer values, extend the range by half a bin on each side of the data extent (binSize = 1 -> extend by 0.5, giving [-2.5, 2.5] for data spanning [-2, 2]).

TestForIssue481.java opens two charts side-by-side: one reproducing the broken behavior and one showing the correct shared-range approach. Issue #481 has been closed with this explanation.