Document sqrt scale mapping and update mapping table

README.md

@@ -48,7 +48,7 @@ The font must be installed at `/usr/local/share/fonts/г/ГОСТ_тип_А.ttf`
 |---|---|
 | Unit | Symbol shown in the left panel (e.g. `U`, `mA`, `%`). Supports inline markup — see below |
 | Range label | Text inside the coloured bar on the right (e.g. `5mA`) |
-| Mapping | **Linear** or **Log**. Log spaces values geometrically (major ticks evenly spaced, minor ticks crowding toward the high end) and requires Min and Max greater than 0 |
+| Mapping | **Linear**, **√** (square root), or **Log**. Major ticks are always evenly spaced physically; the mapping controls their values and how minor ticks are placed within each interval. See below. |
 | Min / Max | Numeric range of the scale |
 | Major intervals | Number of major tick intervals |
 | Minor per interval | Minor ticks between each pair of major ticks |
@@ -58,6 +58,16 @@ The font must be installed at `/usr/local/share/fonts/г/ГОСТ_тип_А.ttf`
 
 Calculated numbers are rendered in fixed-point notation at 4 significant figures — never scientific (e.g. `10000`, not `1e+04`).
 
+### Scale mappings
+
+| Mapping | Major tick values | Minor tick placement | Constraints |
+|---|---|---|---|
+| Linear | Evenly spaced | Uniform | None |
+| √ | Quadratic (0, 1, 4, 9 … × max) — compresses low values | Crowded toward the low end of each interval | Min ≥ 0 |
+| Log | Geometric (min × r⁰, r¹, r² …) — each interval spans the same ratio | Crowded toward the high end of each interval | Min and Max > 0 |
+
+The √ mapping is typical for AC RMS current and power meters, where needle deflection is proportional to the square of the measured quantity.
+
 ### Unit markup
 
 The Unit field accepts a lightweight shorthand (no LaTeX engine — everything is drawn as glyph paths in the ГОСТ тип А font and auto-sized to fit the panel):