Add demo for issue #815 - extra space below yAxisMin

[timmolter]

Closes #815

Why

When a user calls setYAxisMin(0.0), they expect the Y-axis to start exactly at zero with no gap below it. Instead, a visible empty space appears beneath zero. The root cause is that plotContentSize (default ~0.95) computes a symmetric margin on both sides of the axis via Utils.getTickStartOffset(), so the bottom margin persists even after the min is overridden.

Approach

No code change is needed -- the existing API already has a workaround. The fix is to combine three styler calls:

chart.getStyler().setYAxisMin(0.0);
chart.getStyler().setYAxisMax(105.0);   // hardcode a comfortable top bound
chart.getStyler().setPlotContentSize(1.0); // removes the symmetric margin

setPlotContentSize(1.0) makes the tick area fill the full plot height so zero sits exactly at the bottom edge. Because this also removes the top margin, setYAxisMax() to a value slightly above the data max (e.g. 105 when data tops out near 100) restores the breathing room at the top.

Changes

• Added TestForIssue815.java with two getChart*() methods: one reproducing the problem and one showing the fixed output.
• Posted a comment on issue How to remove extra space below min y-axis value? #815 explaining the root cause and the workaround.