fpspreadsheet: Supporting smooth scatter/line series for xlsx (less options than for ods)

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@9219 8e941d3f-bd1b-0410-a28a-d453659cc2b4

components/fpspreadsheet/examples/other/chart/scatter_write_demo.lpr

@@ -111,7 +111,10 @@ begin
            ch.YAxis.Logarithmic := true;
         end;
     end;
-    ch.XAxis.Inverted := true;
+
+    // For testing:
+    // ch.XAxis.Inverted := true;
+    ch.Interpolation := ciCubicSpline;
 
     // Add scatter series
     ser := TsScatterSeries.Create(ch);

components/fpspreadsheet/source/common/xlsxooxmlchart.pas

@@ -3964,14 +3964,25 @@ procedure TsSpreadOOXMLChartWriter.WriteScatterSeries(AStream: TStream;
 var
   indent: String;
   chart: TsChart;
+  scatterStyleStr: String;
 begin
   indent := DupeString(' ', AIndent);
   chart := ASeries.Chart;
 
+  case chart.Interpolation of
+    ciLinear:
+      scatterStyleStr := 'lineMarker';
+    ciCubicSpline, ciBSpline:
+      scatterStyleStr := 'smoothMarker';
+    else
+      //ciStepStart, ciStepEnd, ciCenterX, ciCenterY
+      scatterStyleStr := 'lineMarker';      // better than nothing...
+  end;
+
   AppendToStream(AStream,
     indent + '<c:scatterChart>' + LE +
     indent + '  <c:varyColors val="0"/>' + LE +
-    indent + '  <c:scatterStyle val="lineMarker"/>' + LE
+    indent + '  <c:scatterStyle val="' + scatterStyleStr + '"/>' + LE
   );
 
   WriteChartSeriesNode(AStream, AIndent + 4, ASeries, ASeriesIndex);