Fix blank ride plot in compare mode

.. when you switch tabs to ride plot having never
   displayed any ride at all.

.. also fixes switching to compare mode when the currently
   selected ride is blank (has no data).

src/AllPlotWindow.cpp

@@ -680,6 +680,14 @@ AllPlotWindow::compareChanged()
     // new ones ..
     if (context->isCompareIntervals) {
 
+        // first, lets init fullPlot, just in case its never
+        // been set (ie, switched to us before ever plotting a ride
+        if (myRideItem) fullPlot->setDataFromRide(myRideItem);
+
+        // and even if the current ride is blank, we're not
+        // going to be blank !!
+        setIsBlank(false);
+
         //
         // SETUP FULLPLOT FOR COMPARE MODE
         //