vg/vgimg: handle dash offsets

Updates fogleman/gg#64.

go.mod

@@ -2,7 +2,7 @@ module gonum.org/v1/plot
 
 require (
 	github.com/ajstarks/svgo v0.0.0-20180226025133-644b8db467af
-	github.com/fogleman/gg v1.2.0
+	github.com/fogleman/gg v1.2.1-0.20190220221249-0403632d5b90
 	github.com/golang/freetype v0.0.0-20170609003504-e2365dfdc4a0
 	github.com/jung-kurt/gofpdf v1.0.0
 	golang.org/x/exp v0.0.0-20180807140117-3d87b88a115f

go.sum

@@ -1,7 +1,7 @@
 github.com/ajstarks/svgo v0.0.0-20180226025133-644b8db467af h1:wVe6/Ea46ZMeNkQjjBW6xcqyQA/j5e0D6GytH95g0gQ=
 github.com/ajstarks/svgo v0.0.0-20180226025133-644b8db467af/go.mod h1:K08gAheRH3/J6wwsYMMT4xOr94bZjxIelGM0+d/wbFw=
-github.com/fogleman/gg v1.2.0 h1:Z0uOlqo+EbPQwdbrpKV1/jEcefXGPICDtGmS/gwly30=
-github.com/fogleman/gg v1.2.0/go.mod h1:R/bRT+9gY/C5z7JzPU0zXsXHKM4/ayA+zqcVNZzPa1k=
+github.com/fogleman/gg v1.2.1-0.20190220221249-0403632d5b90 h1:WXb3TSNmHp2vHoCroCIB1foO/yQ36swABL8aOVeDpgg=
+github.com/fogleman/gg v1.2.1-0.20190220221249-0403632d5b90/go.mod h1:R/bRT+9gY/C5z7JzPU0zXsXHKM4/ayA+zqcVNZzPa1k=
 github.com/golang/freetype v0.0.0-20170609003504-e2365dfdc4a0 h1:DACJavvAHhabrF08vX0COfcOBJRhZ8lUbR+ZWIs0Y5g=
 github.com/golang/freetype v0.0.0-20170609003504-e2365dfdc4a0/go.mod h1:E/TSTwGwJL78qG/PmXZO1EjYhfJinVAhrmmHX6Z8B9k=
 github.com/jung-kurt/gofpdf v1.0.0 h1:EroSdlP9BOoL5ssLYf3uLJXhCQMMM2fFxCJDKA3RhnA=

vg/vgimg/vgimg.go

@@ -195,11 +195,7 @@ func (c *Canvas) SetLineDash(ds []vg.Length, offs vg.Length) {
 	for i, d := range ds {
 		dashes[i] = d.Dots(c.DPI())
 	}
-	// FIXME(sbinet): handle offs (the distance into the dash pattern
-	// to start the dash.)
-	// this needs to be implemented on the gg side.
-	// see:
-	//  - https://github.com/fogleman/gg/issues/64
+	c.ctx.SetDashOffset(offs.Dots(c.DPI()))
 	c.ctx.SetDash(dashes...)
 }