plot: add hovertemplate to ScatterTrace (#233)

* plot: add hovertemplate to ScatterTrace

adds a hovertemplate string @[omitempty] field to ScatterTrace in the plot module.
Several trace types already expose Plotly’s hovertemplate (for example HeatmapTrace, ContourTrace, SurfaceTrace, ChoroplethTrace, ScatterMapboxTrace, and SankeyLink). This change brings the scatter trace in line with those APIs and with Plotly’s own scatter trace options.
A small test (test_scatter_hovertemplate_in_traces_json) verifies that the hovertemplate field is emitted in the traces JSON when set.

* v fmt

* make hovertemplate test less brittle to json changes

Author: holder66

plot/plot_test.v

@@ -12,3 +12,16 @@ fn test_bar() {
 	)
 	plt.show()!
 }
+
+fn test_scatter_hovertemplate_in_json() {
+	mut plt := Plot.new()
+	mut tr := ScatterTrace{
+		x:             [1.0]
+		y:             [2.0]
+		hovertemplate: 'x=%{x}, y=%{y}<extra></extra>'
+	}
+	plt.add_trace(tr)
+	traces, _ := plt.to_json()
+	assert traces.contains('"hovertemplate":')
+	assert traces.contains('x=%{x}, y=%{y}<extra></extra>')
+}

plot/trace.v

@@ -68,6 +68,8 @@ pub mut:
 @[params]
 pub struct ScatterTrace {
 	CommonTrace
+pub mut:
+	hovertemplate string @[omitempty]
 }
 
 // PieTrace is a struct for Pie trace type