Fix LTM legend colours

.. when user changes the background color for charts the
   legend can be difficult to read as it is always in black.

.. we now update the legend and it's widgets to change the text
   color when the chart is refreshed.

src/LTMPlot.cpp

@@ -150,7 +150,14 @@ LTMPlot::configUpdate()
         axisWidget(x)->setPalette(palette);
     }
     axisWidget(QwtPlot::xBottom)->setPalette(palette);
-    this->legend()->setPalette(palette);
+    QwtLegend *l = static_cast<QwtLegend *>(this->legend());
+    l->setPalette(palette);
+    foreach(QwtPlotCurve *p, curves) {
+        foreach (QWidget *w, l->legendWidgets(itemToInfo(p))) {
+            w->setPalette(palette);
+        }
+    }
+    updateLegend();
 }
 
 void
@@ -1129,6 +1136,9 @@ LTMPlot::setData(LTMSettings *set)
 
     //qDebug()<<"Final tidy.."<<timer.elapsed();
 
+    // update colours etc for plot chrome
+    configUpdate();
+
     // plot
     replot();