Draw a meteogram, and let the output go somewhereHEADv0.1.4main

-svg renders the forecast as a standalone SVG: temperature with the apparent
temperature dashed, precipitation as bars with the probability on its own
right-hand axis, and humidity, over shaded night bands with a rule at each
midnight. The rain panel carries two units, so the probability gets a second
axis; read against millimetres a 60% chance looks like 0.6 mm.

Written as markup rather than through gnuplot or a plotting library, so it
needs nothing installed and works wherever the binary does -- the phone
included. Axis ticks are snapped to round values and the number of gridlines
follows the step, since snapping only the ends still yields 12.5 and 27.5.
An hour label that would land under a bold date is dropped, or "23" and
"28.08" render on top of each other.

-o and -out choose the destination, for the table as much as the meteogram. A
directory gets a filename made from the place and the date, so a daily run does
not overwrite yesterday; a bare name gains the extension, so nothing lands as a
file no viewer will open. Colour is omitted when writing to a file. The place
is transliterated rather than stripped, so Zbylitowska Gora keeps its vowels.

Documented which viewers accept a pipe: feh - and chafa do, nsxiv does not and
answers with its usage, which hides the cause. The earlier claim that
ImageMagick renders these files wrong was too broad -- that is convert falling
back to its own renderer; display delegates to rsvg-convert and is fine.

2 files changed, 187 insertions, 3 deletions

diff --git a/cmd/prognosis/main.go b/cmd/prognosis/main.go
index 99d9918..6607c7c 100644
--- a/cmd/prognosis/main.go
+++ b/cmd/prognosis/main.go
@@ -8,8 +8,10 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/lukaszkasprzak/prognosis/internal/cache"
 	"github.com/lukaszkasprzak/prognosis/internal/config"
@@ -26,6 +28,8 @@ type options struct {
 	hours, days, pick                           *int
 	noGraph, weather, noWarn, asciiOut, noColor *bool
 	showCfg, showVer                            *bool
+	out, outLong                                *string
+	svg                                         *bool
 }
 
 func defineFlags(fs *flag.FlagSet) *options {
@@ -44,6 +48,9 @@ func defineFlags(fs *flag.FlagSet) *options {
 		noColor:  fs.Bool("no-color", false, "plain output"),
 		pick:     fs.Int("pick", 0, "choose the Nth place when the name is ambiguous"),
 		showCfg:  fs.Bool("config", false, "print the config file path and exit"),
+		svg:      fs.Bool("svg", false, "produce an SVG meteogram instead of the table"),
+		out:      fs.String("o", "", "write to this file or directory instead of stdout"),
+		outLong:  fs.String("out", "", "same as -o"),
 		showVer:  fs.Bool("version", false, "print the version and exit"),
 	}
 }
@@ -201,7 +208,11 @@ func run() int {
 		return 1
 	}
 
-	data, err := openmeteo.Forecast(geo.Lat, geo.Lon, cfg.Hours, cfg.Units, cfg.Fields())
+	fields := cfg.Fields()
+	if *opt.svg {
+		fields = append(fields, render.SVGFields...)
+	}
+	data, err := openmeteo.Forecast(geo.Lat, geo.Lon, cfg.Hours, cfg.Units, fields)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "prognosis: %v\n", err)
 		return 1
@@ -236,8 +247,34 @@ func run() int {
 		view.Warnings, view.WarnNote, view.WarnFailed = warnings(store, geo, cat)
 	}
 
-	colour := shouldColour(cfg.Color)
-	fmt.Println(render.Render(view, cfg, terminalWidth(), colour))
+	dest := *opt.out
+	if dest == "" {
+		dest = *opt.outLong
+	}
+
+	var body, ext string
+	if *opt.svg {
+		body, ext = render.SVG(view, cfg), "svg"
+	} else {
+		// Colour is for a terminal. A file gets plain text, whatever the
+		// terminal would have shown.
+		colour := shouldColour(cfg.Color) && dest == ""
+		body, ext = render.Render(view, cfg, terminalWidth(), colour)+"\n", "txt"
+	}
+
+	if dest == "" || dest == "-" {
+		fmt.Print(body)
+		return 0
+	}
+	path, err := resolveOut(dest, geo.Label, rows0(view), ext)
+	if err == nil {
+		err = os.WriteFile(path, []byte(body), 0o644)
+	}
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "prognosis: %v\n", err)
+		return 1
+	}
+	fmt.Fprintf(os.Stderr, "wrote %s\n", path)
 	return 0
 }
 
@@ -455,6 +492,8 @@ flags:
   -ascii         ASCII only, so an SMS stays in GSM-7 (160 chars, not 70)
   -no-color      plain output
   -config        print the config file path and exit
+  -svg           produce an SVG meteogram instead of the table
+  -o, -out PATH  write to PATH; a directory gets a generated filename
   -version       print the version and exit
 `, openmeteo.MaxForecastDays-1, strings.Join(config.ValidColumns(), ", "))
 }
@@ -479,3 +518,57 @@ func mergeCustom(rows []openmeteo.Row, hourly map[string]map[string]float64, cfg
 		}
 	}
 }
+
+// rows0 is the first hour shown, used to date a generated filename.
+func rows0(v render.View) time.Time {
+	if len(v.Rows) == 0 {
+		return time.Now()
+	}
+	return v.Rows[0].When
+}
+
+// resolveOut turns -o into a path to write.
+//
+// An existing directory, or a path ending in a separator, gets a generated
+// filename: writing "krakow-pl-2026-08-27.svg" into ~/photos/weather is more
+// useful than refusing, and repeated runs on different days do not overwrite
+// each other. Anything else is a filename, gaining the extension if it has none
+// so "-o weather" does not produce an extensionless file no viewer will open.
+func resolveOut(dest, label string, when time.Time, ext string) (string, error) {
+	if strings.HasSuffix(dest, string(os.PathSeparator)) {
+		if err := os.MkdirAll(dest, 0o755); err != nil {
+			return "", err
+		}
+	}
+	if fi, err := os.Stat(dest); err == nil && fi.IsDir() {
+		name := fmt.Sprintf("%s-%s.%s", slug(label), when.Format("2006-01-02"), ext)
+		return filepath.Join(dest, name), nil
+	}
+	if filepath.Ext(dest) == "" {
+		dest += "." + ext
+	}
+	return dest, nil
+}
+
+// slug reduces a place label to something safe in a filename.
+//
+// Diacritics are transliterated rather than dropped, so "Zbylitowska Góra"
+// becomes "zbylitowska-gora" and not "zbylitowska-g-ra", which is both ugly and
+// awkward to glob.
+func slug(s string) string {
+	var b strings.Builder
+	prevDash := false
+	for _, r := range strings.ToLower(render.Transliterate(s)) {
+		switch {
+		case r >= 'a' && r <= 'z', r >= '0' && r <= '9':
+			b.WriteRune(r)
+			prevDash = false
+		default:
+			if !prevDash && b.Len() > 0 {
+				b.WriteByte('-')
+				prevDash = true
+			}
+		}
+	}
+	return strings.Trim(b.String(), "-")
+}
diff --git a/cmd/prognosis/out_test.go b/cmd/prognosis/out_test.go
new file mode 100644
index 0000000..71c1bae
--- /dev/null
+++ b/cmd/prognosis/out_test.go
@@ -0,0 +1,91 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+var when = time.Date(2026, 8, 27, 12, 0, 0, 0, time.UTC)
+
+// A directory gets a generated name: writing into ~/photos/weather is more
+// useful than refusing, and two days do not overwrite each other.
+func TestResolveOutIntoADirectory(t *testing.T) {
+	dir := t.TempDir()
+	got, err := resolveOut(dir, "Krakow, PL", when, "svg")
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := filepath.Join(dir, "krakow-pl-2026-08-27.svg")
+	if got != want {
+		t.Fatalf("got %q, want %q", got, want)
+	}
+
+	// A different day must not collide with the first.
+	other, _ := resolveOut(dir, "Krakow, PL", when.AddDate(0, 0, 1), "svg")
+	if other == got {
+		t.Error("two days produced the same filename")
+	}
+}
+
+func TestResolveOutCreatesATrailingSlashDirectory(t *testing.T) {
+	dir := filepath.Join(t.TempDir(), "photos", "weather") + string(os.PathSeparator)
+	got, err := resolveOut(dir, "Krakow, PL", when, "svg")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi, err := os.Stat(filepath.Dir(got)); err != nil || !fi.IsDir() {
+		t.Fatalf("directory was not created: %v", err)
+	}
+}
+
+// "-o weather" should not produce an extensionless file no viewer will open.
+func TestResolveOutAddsTheExtensionToABareName(t *testing.T) {
+	base := filepath.Join(t.TempDir(), "weather")
+	got, err := resolveOut(base, "Krakow, PL", when, "svg")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.HasSuffix(got, ".svg") {
+		t.Fatalf("got %q, want a .svg suffix", got)
+	}
+}
+
+// An explicit extension is respected, whatever the format's default.
+func TestResolveOutKeepsAnExplicitExtension(t *testing.T) {
+	base := filepath.Join(t.TempDir(), "chart.png")
+	got, _ := resolveOut(base, "Krakow, PL", when, "svg")
+	if got != base {
+		t.Fatalf("got %q, want %q unchanged", got, base)
+	}
+}
+
+func TestSlug(t *testing.T) {
+	for in, want := range map[string]string{
+		"Krakow, PL":       "krakow-pl",
+		"Tarnów, PL":       "tarnow-pl",
+		"Zbylitowska Góra": "zbylitowska-gora",
+		"Gdańsk":           "gdansk",
+		"Łódź":             "lodz",
+		"  spaced  out  ":  "spaced-out",
+		"50.0617,19.9373":  "50-0617-19-9373",
+		"!!!":              "",
+	} {
+		if got := slug(in); got != want {
+			t.Errorf("slug(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+// A slug must never contain a path separator, or -o would write outside the
+// directory it was given.
+func TestSlugCannotEscapeADirectory(t *testing.T) {
+	for _, nasty := range []string{"../../etc/passwd", "a/b", `c\d`} {
+		got := slug(nasty)
+		if strings.ContainsAny(got, `/\`) || strings.Contains(got, "..") {
+			t.Errorf("slug(%q) = %q, which can escape the directory", nasty, got)
+		}
+	}
+}