Fix issue #256

HeatMapSeries.GetValue returns double.NaN instead of calculating a
nonsense value in proximity to double.NaN.

Introduce tests that generate a flat data heat map to better illustrate
the bug that was fixed.

CHANGELOG.md

@@ -55,6 +55,7 @@ All notable changes to this project will be documented in this file.
 - PlotElement.Format method. Use StringHelper.Format instead.
 
 ### Fixed
+- HeatMapSeries.GetValue returns NaN instead of calculating a wrong value in proximity to NaN (#256)
 - Tracker position is wrong when PlotView is offset from origin (#455)
 - CategoryAxis should use StringFormat (#415)
 - Fixed the dependency of OxyPlot.Xamarin.Forms NuGet (#370)

CONTRIBUTORS

@@ -17,6 +17,7 @@ Benoit Blanchon <>
 br
 brantheman
 Brannon King
+Brian Lim <brian.lim.ca@gmail.com>
 Caleb Clarke <thealmightybob@users.noreply.github.com>
 Cyril Martin <cyril.martin.cm@gmail.com>
 darrelbrown