Add label name to Plotter CHECK.

Signed-off-by: Austin Schuh <austin.linux@gmail.com>
Change-Id: I035344573cbee5a021a4e816186438a7590c5e18
diff --git a/frc971/analysis/in_process_plotter.cc b/frc971/analysis/in_process_plotter.cc
index 33d999e..b327c54 100644
--- a/frc971/analysis/in_process_plotter.cc
+++ b/frc971/analysis/in_process_plotter.cc
@@ -80,7 +80,7 @@
 
 void Plotter::AddLine(const std::vector<double> &x,
                       const std::vector<double> &y, LineOptions options) {
-  CHECK_EQ(x.size(), y.size());
+  CHECK_EQ(x.size(), y.size()) << ": " << options.label;
   CHECK(!position_.IsNull())
       << "You must call AddFigure() before calling AddLine().";