Re #11422 Improve appearance of plot when n is -ve

Code/Mantid/MantidPlot/src/Graph.cpp

@@ -1308,6 +1308,18 @@ void Graph::setAxisScale(int axis, double start, double end, int type, double st
   if (type == GraphOptions::Log10)
   {
     sc_engine->setType(ScaleTransformation::Log10);
+  }
+  else if (type == GraphOptions::Power)
+  {
+    sc_engine->setType(ScaleTransformation::Power);
+  }
+  else
+  {
+    sc_engine->setType(ScaleTransformation::Linear);
+  }
+
+  if (type == GraphOptions::Log10 || type == GraphOptions::Power)
+  {
     if (start <= 0)
     {
       double s_min = DBL_MAX;
@@ -1353,14 +1365,6 @@ void Graph::setAxisScale(int axis, double start, double end, int type, double st
     // log scales can't represent zero or negative values, 1e-10 is a low number that I hope will be lower than most of the data but is still sensible for many color plots
     //start = start < 1e-90 ? 1e-10 : start;
   }
-  else if (type == GraphOptions::Power)
-  {
-    sc_engine->setType(ScaleTransformation::Power);
-  }
-  else
-  {
-    sc_engine->setType(ScaleTransformation::Linear);
-  }
 
   if (axis == QwtPlot::yRight)
   {

Code/Mantid/MantidPlot/src/plot2D/PowerScaleEngine.cpp

@@ -141,6 +141,16 @@ void PowerScaleEngine::buildTicks(
     for ( int i = 0; i < QwtScaleDiv::NTickTypes; i++ )
     {
         ticks[i] = strip(ticks[i], interval);
+
+        // ticks very close to 0.0 are
+        // explicitly set to 0.0
+        // important if nth_power is set to -ve value
+
+        for ( int j = 0; j < (int)ticks[i].count(); j++ )
+        {
+            if ( QwtScaleArithmetic::compareEps(ticks[i][j], 0.0, stepSize) == 0 )
+                ticks[i][j] = 0.0;
+        }
     }
 }