Overview Sparkline Y-axis Fixup

.. just use min/max -/+ 10% of difference between the 2

src/Charts/OverviewWindow.cpp

@@ -380,7 +380,8 @@ Card::setData(RideItem *item)
                 if (v>max) max=v;
                 if (v<min) min=v;
             }
-            //chart->axisY(lineseries)->setRange(min,max); //XXX need to fix axis calculation
+            double diff = (max-min)/10.0f;
+            chart->axisY(lineseries)->setRange(min-diff,max+diff); // add 10% to each direction
         }
     }