Plotly extension method does not work in .csx

[gyurisc]

The following errors are happening when trying to call Svg or Png extension methods:

+// (9,1): error CS1929: 'PlotlyChart' does not contain a definition for 'Png' and the best extension method overload 'Png(PlotlyChart)' requires a receiver of type 'PlotlyChart'
+public static BinaryOutput Png(this PlotlyChart chart) =>

• new BinaryOutput() { ContentType = "text/html", Data = chart.GetPngHtml() };

+// (9,1): error CS1929: 'PlotlyChart' does not contain a definition for 'Svg' and the best extension method overload 'Svg(PlotlyChart)' requires a receiver of type 'PlotlyChart'
+public static BinaryOutput Svg(this PlotlyChart chart) =>

• new BinaryOutput() { ContentType = "text/html", Data = chart.GetSvgHtml() };