fix tickFormat on a collapsed domain (start == end)

closes #286
closes observablehq/plot#2076

Author: Fil

src/tickFormat.js

@@ -1,10 +1,10 @@
 import {tickStep} from "d3-array";
 import {format, formatPrefix, formatSpecifier, precisionFixed, precisionPrefix, precisionRound} from "d3-format";
 
-export default function tickFormat(start, stop, count, specifier) {
+export default function tickFormat(start, stop, count, specifierIn) {
   var step = tickStep(start, stop, count),
       precision;
-  specifier = formatSpecifier(specifier == null ? ",f" : specifier);
+  const specifier = formatSpecifier(specifierIn == null ? ",f" : specifierIn);
   switch (specifier.type) {
     case "s": {
       var value = Math.max(Math.abs(start), Math.abs(stop));
@@ -21,7 +21,14 @@ export default function tickFormat(start, stop, count, specifier) {
     }
     case "f":
     case "%": {
-      if (specifier.precision == null && !isNaN(precision = precisionFixed(step))) specifier.precision = precision - (specifier.type === "%") * 2;
+      if (specifier.precision == null) {
+        if (step === 0) {
+          if (specifierIn == null) specifier.trim = true;
+        }
+        else if (!isNaN(precision = precisionFixed(step))) {
+          specifier.precision = precision - (specifier.type === "%") * 2;
+        }
+      } 
       break;
     }
   }

test/tickFormat-test.js

@@ -40,4 +40,20 @@ it("tickFormat(start, stop, count) uses the default precision when the domain is
   const f = tickFormat(0, NaN, 10);
   assert.strictEqual(f + "", " >-,f");
   assert.strictEqual(f(0.12), "0.120000");
+
+  const f2 = tickFormat(0.12, NaN, 10);
+  assert.strictEqual(f2 + "", " >-,f");
+  assert.strictEqual(f2(0.12), "0.120000");
+});
+
+it("tickFormat(start, stop, count) uses the default precision with trimming when the domain is collapsed", () => {
+  const f = tickFormat(5.5, 5.5, 10);
+  assert.strictEqual(f + "", " >-,~f");
+  assert.strictEqual(f(0), "0");
+  assert.strictEqual(f(5.5), "5.5");
+  assert.strictEqual(f(Math.PI), "3.141593");
+  assert.strictEqual(f(-Math.PI), "−3.141593");
+  assert.strictEqual(f(Math.PI * 1e7), "31,415,926.535898");
+  assert.strictEqual(f(-Math.PI * 1e7), "−31,415,926.535898");
 });
+