plot line: sort points by X for plotting lines. fix #280

Author: shenwei356

CHANGELOG.md

@@ -21,6 +21,7 @@
         - new flag `-b, --buffer-size`.
     - `csvtk plot line`:
         - only add legend for more than one group. [#279](https://github.com/shenwei356/csvtk/issues/279)
+        - sort points by X for plotting lines. [#280](https://github.com/shenwei356/csvtk/issues/280)
 - [csvtk v0.30.0](https://github.com/shenwei356/csvtk/releases/tag/v0.30.0)
 [![Github Releases (by Release)](https://img.shields.io/github/downloads/shenwei356/csvtk/v0.30.0/total.svg)](https://github.com/shenwei356/csvtk/releases/tag/v0.30.0)
   - `csvtk`:

csvtk/cmd/line.go

@@ -194,6 +194,14 @@ Notes:
 		addLegend := len(groupOrders) > 1
 		for _, gor := range groupOrders {
 			v := groups[gor.Key]
+
+			// sort by x
+			if !scatter {
+				sort.Slice(v, func(i, j int) bool {
+					return v[i].X < v[j].X
+				})
+			}
+
 			g := gor.Key
 			if !scatter {
 				lines, points, err := plotter.NewLinePoints(v)