Update logarithmicScale (used when forceNiceScale=false) to respect yMin andyMax limits without changing them, and provide a useful log scale between them.

Author: David Alban

src/modules/Scales.js

@@ -183,22 +183,40 @@ export default class Range {
   }
 
   logarithmicScale(yMin, yMax, base) {
+    // Basic validation to avoid for loop starting at -inf.
+    if (yMax <= 0) yMax = Math.max(yMin, base)
+    if (yMin <= 0) yMin = Math.min(yMax, base)
+
     const logs = []
 
-    const ticks = Math.ceil(Math.log(yMax) / Math.log(base)) + 1 // Get powers of base up to our max, and then one more
+    // Get the logarithmic range.
+    const logMax = Math.log(yMax) / Math.log(base)
+    const logMin = Math.log(yMin) / Math.log(base)
 
-    for (let i = 0; i < ticks; i++) {
-      logs.push(Math.pow(base, i))
-    }
+    // Get the exact logarithmic range.
+    // (This is the exact number of multiples of the base there are between yMin and yMax).
+    const logRange = logMax - logMin
 
-    if (yMin === 0) {
-      logs.unshift(yMin)
+    // Round the logarithmic range to get the number of ticks we will create.
+    // If the chosen min/max values are multiples of each other WRT the base, this will be neat.
+    // If the chosen min/max aren't, we will at least still provide USEFUL ticks.
+    const ticks = Math.round(logRange)
+
+    // Get the logarithmic spacing between ticks.
+    const logTickSpacing = logRange / ticks
+
+    // Create as many ticks as there is range in the logs.
+    for (let i = 0, logTick = logMin; i < ticks; i++, logTick += logTickSpacing) {
+      logs.push(Math.pow(base, logTick))
     }
 
+    // Add a final tick at the yMax.
+    logs.push(Math.pow(base, logMax))
+
     return {
       result: logs,
-      niceMin: logs[0],
-      niceMax: logs[logs.length - 1]
+      niceMin: yMin,
+      niceMax: yMax
     }
   }