skip buffer by using io.WriterTo directly

main.go

@@ -6,7 +6,6 @@ package main
 import (
 	"flag"
 	"fmt"
-	"io"
 	"os"
 	"runtime"
 	"time"
@@ -104,7 +103,7 @@ func main() {
 			out, err = os.Create(*file)
 			fatal(err, "failed to create file")
 		}
-		_, err = io.Copy(out, plot)
+		_, err = plot.WriteTo(out)
 		fatal(err, "failed to copy to file")
 
 		// Upload to Slack

promplot/plot.go

@@ -1,7 +1,6 @@
 package promplot
 
 import (
-	"bytes"
 	"fmt"
 	"io"
 	"regexp"
@@ -20,7 +19,7 @@ var labelText = regexp.MustCompile("\\{(.*)\\}")
 
 // Plot creates a plot from metric data and saves it to a temporary file.
 // It's the callers responsibility to remove the returned file when no longer needed.
-func Plot(metrics model.Matrix, title, format string) (io.Reader, error) {
+func Plot(metrics model.Matrix, title, format string) (io.WriterTo, error) {
 	p, err := plot.New()
 	if err != nil {
 		return nil, fmt.Errorf("failed to create new plot: %v", err)
@@ -90,10 +89,5 @@ func Plot(metrics model.Matrix, title, format string) (io.Reader, error) {
 	}
 	p.Draw(draw.Crop(draw.New(c), margin, -margin, margin, -margin))
 
-	b := new(bytes.Buffer)
-	if _, err = c.WriteTo(b); err != nil {
-		return nil, fmt.Errorf("failed to save plot: %v", err)
-	}
-
-	return b, nil
+	return c, nil
 }

promplot/slack.go

@@ -10,7 +10,7 @@ import (
 )
 
 // Slack posts a file to a Slack channel.
-func Slack(token, channel, title string, plot io.Reader) error {
+func Slack(token, channel, title string, plot io.WriterTo) error {
 	api := slack.New(token)
 
 	_, _, err := api.PostMessage(channel, title, slack.PostMessageParameters{
@@ -35,9 +35,9 @@ func Slack(token, channel, title string, plot io.Reader) error {
 			panic(fmt.Errorf("failed to delete tmp file: %v", err))
 		}
 	}()
-	_, err = io.Copy(f, plot)
+	_, err = plot.WriteTo(f)
 	if err != nil {
-		return fmt.Errorf("failed to copy plot to file: %v", err)
+		return fmt.Errorf("failed to write plot to file: %v", err)
 	}
 
 	_, err = api.UploadFile(slack.FileUploadParameters{