Fallback to linear ticks for small domains.

Fixes #44.
Fixes #49.
Fixes #129.

src/log.js

@@ -86,22 +86,23 @@ export function loggish(transform) {
         z = [];
 
     if (!(base % 1) && j - i < n) {
-      i = Math.round(i) - 1, j = Math.round(j) + 1;
-      if (u > 0) for (; i < j; ++i) {
+      i = Math.floor(i), j = Math.ceil(j);
+      if (u > 0) for (; i <= j; ++i) {
         for (k = 1, p = pows(i); k < base; ++k) {
           t = p * k;
           if (t < u) continue;
           if (t > v) break;
           z.push(t);
         }
-      } else for (; i < j; ++i) {
+      } else for (; i <= j; ++i) {
         for (k = base - 1, p = pows(i); k >= 1; --k) {
           t = p * k;
           if (t < u) continue;
           if (t > v) break;
           z.push(t);
         }
       }
+      if (!z.length) z = ticks(u, v, n);
     } else {
       z = ticks(i, j, Math.min(j - i, n)).map(pows);
     }

test/log-test.js

@@ -299,6 +299,14 @@ tape("log.ticks() generates the expected power-of-ten ticks for small domains",
   test.end();
 });
 
+tape("log.ticks() generates linear ticks when the domain extent is small", function(test) {
+  var s = scale.scaleLog();
+  test.deepEqual(s.domain([41, 42]).ticks(), [41, 41.1, 41.2, 41.3, 41.4, 41.5, 41.6, 41.7, 41.8, 41.9, 42]);
+  test.deepEqual(s.domain([42, 41]).ticks(), [42, 41.9, 41.8, 41.7, 41.6, 41.5, 41.4, 41.3, 41.2, 41.1, 41]);
+  test.deepEqual(s.domain([1600, 1400]).ticks(), [1600, 1580, 1560, 1540, 1520, 1500, 1480, 1460, 1440, 1420, 1400]);
+  test.end();
+});
+
 tape("log.base(base).ticks() generates the expected power-of-base ticks", function(test) {
   var s = scale.scaleLog().base(Math.E);
   test.deepEqual(s.domain([0.1, 100]).ticks().map(round), [0.135335283237, 0.367879441171, 1, 2.718281828459, 7.389056098931, 20.085536923188, 54.598150033144]);