[birt-dev] Checkin: Bugzilla#194104 - Chart Preview is blank for Differe

["Yulin Wang" <yulin.wang@xxxxxxxxxxx>]

- Summary:
Chart Preview is blank for Difference chart in Edit Chart dialog

- Bugzilla Bug (s) Resolved: 194104

- Description:
Check in #194104 to HEAD
Caused by too few data points (less than 1) won’t generate curve key points but return null.
Fixed an internal NPE in Difference chart. This exception is only visible after enabling tracing.

- Tests Description: Manual Test

- Notes to Build Team: N/A

- Notes to Developers: N/A

- Notes to QA: N/A

- Notes to Documentation: N/A

- Files Edited:

/org.eclipse.birt.chart.engine.extension/src/org/eclipse/birt/chart/render/DifferenceRenderer.java

- Files Added:

- Files Deleted