Fix #805: wrong value displayed in cursor label

[timmolter]

Fixes #805

Problem

Two bugs in Cursor.calculateMatchingDataPoints():

1. Stale tooltip values — matchingDataPointList was only cleared inside the if (dataPoints.size() > 0) branch. When the cursor moved to a position where no data points were nearby (but still inside the plot), the old tooltip kept showing from the previous cursor position.

2. Missing Y values for same-series multi-point hits — The HashMap deduplication kept only one DataPoint per series (the closest X), silently dropping other Y values at the same X position. This caused wrong/incomplete values in the cursor label when multiple points of the same series fell under the cursor.

Fix

• matchingDataPointList.clear() is now called unconditionally at the start of calculateMatchingDataPoints(), so no stale values persist when the cursor is between data points.
• Replaced the HashMap closest-X logic with a LinkedHashMap that accumulates all Y values for each series and joins them with ", ", so all values are visible in the cursor label (e.g. "1.0, 2.0"). Series insertion order is preserved.

Demo

TestForIssue805.java — two overlapping series on an XY chart; enable the cursor and hover across the chart to verify correct labels.

[timmolter]

Superseded by a more complete fix — closing in favour of a new PR.