better graphing

HOWTO.md

@@ -402,6 +402,10 @@ Each graph contains a filled temperature area, a solid humidity line, and a
 dotted battery line. Initially, most of each graph is blank because no historic
 samples existed before the RRD was created.
 
+Temperature uses the left axis with a default range of 0–30°C. Humidity and
+battery use the right axis with a default range of 0–100%. These ranges and
+labels can be changed in `config.yaml` under `graph.axes`.
+
 The collector also generates `/var/www/html/sensors/index.html`. It displays
 six graphs at a time and has controls for switching between the daily, monthly,
 and two-year periods. It remembers the selected period and refreshes the PNGs

config.yaml

@@ -72,9 +72,16 @@ graph:
   width: 1000
   height: 360
   end: now
-  vertical_label: degrees C / percent
   slope_mode: true
-  alt_autoscale: true
+  axes:
+    temperature:
+      minimum: 0
+      maximum: 30
+      label: degrees C
+    percentage:
+      minimum: 0
+      maximum: 100
+      label: percent
   periods:
     day:
       start: end-1d

read_temperature.py

@@ -194,11 +194,26 @@ def render_graphs(sensor: Sensor, rrd_path: Path, config: Config) -> None:
     temperature = graph["temperature"]
     humidity = graph["humidity"]
     battery = graph["battery"]
+    temperature_axis = graph["axes"]["temperature"]
+    percentage_axis = graph["axes"]["percentage"]
+    temperature_minimum = float(temperature_axis["minimum"])
+    temperature_maximum = float(temperature_axis["maximum"])
+    percentage_minimum = float(percentage_axis["minimum"])
+    percentage_maximum = float(percentage_axis["maximum"])
+    temperature_span = temperature_maximum - temperature_minimum
+    percentage_span = percentage_maximum - percentage_minimum
+    if temperature_span <= 0 or percentage_span <= 0:
+        raise SystemExit("Graph axis maximums must be greater than their minimums")
+
+    percentage_to_temperature = temperature_span / percentage_span
+    right_axis_scale = percentage_span / temperature_span
+    right_axis_shift = percentage_minimum - temperature_minimum * right_axis_scale
+    percentage_transform = (
+        f"{percentage_minimum},-,{percentage_to_temperature},*,{temperature_minimum},+"
+    )
     optional_flags = []
     if graph.get("slope_mode", False):
         optional_flags.append("--slope-mode")
-    if graph.get("alt_autoscale", False):
-        optional_flags.append("--alt-autoscale")
 
     for suffix, period in graph["periods"].items():
         output = config.graph_dir / f"{sensor.slug}_{suffix}.png"
@@ -206,15 +221,22 @@ def render_graphs(sensor: Sensor, rrd_path: Path, config: Config) -> None:
             "graph", str(output), "--start", str(period["start"]),
             "--end", str(graph["end"]),
             "--title", f"{sensor.name} - {period['label']}",
-            "--vertical-label", str(graph["vertical_label"]),
+            "--vertical-label", str(temperature_axis["label"]),
+            "--right-axis", f"{right_axis_scale}:{right_axis_shift}",
+            "--right-axis-label", str(percentage_axis["label"]),
+            "--lower-limit", str(temperature_minimum),
+            "--upper-limit", str(temperature_maximum),
+            "--rigid",
             "--width", str(graph["width"]), "--height", str(graph["height"]),
             *optional_flags,
             f"DEF:temp={rrd_path}:temperature:AVERAGE",
             f"DEF:humidity={rrd_path}:humidity:AVERAGE",
             f"DEF:battery={rrd_path}:battery:AVERAGE",
+            f"CDEF:humidity_scaled=humidity,{percentage_transform}",
+            f"CDEF:battery_scaled=battery,{percentage_transform}",
             f"AREA:temp#{temperature['color']}:{temperature['legend']}",
-            f"LINE{humidity['line_width']}:humidity#{humidity['color']}:{humidity['legend']}",
+            f"LINE{humidity['line_width']}:humidity_scaled#{humidity['color']}:{humidity['legend']}",
-            f"LINE{battery['line_width']}:battery#{battery['color']}:{battery['legend']}:dashes={battery['dashes']}",
+            f"LINE{battery['line_width']}:battery_scaled#{battery['color']}:{battery['legend']}:dashes={battery['dashes']}",
             f"GPRINT:temp:LAST:{temperature['last_value_format']}",
             f"GPRINT:humidity:LAST:{humidity['last_value_format']}",
             f"GPRINT:battery:LAST:{battery['last_value_format']}",