Interpolate contours in linear space

Author: andrew-matteson

src/traces/contour/find_all_paths.js

@@ -269,16 +269,25 @@ function getInterpPx(pi, loc, step) {
     var xa = pi.xaxis;
     var ya = pi.yaxis;
 
+    // Interpolate in linear space, then convert to pixel
     if(step[1]) {
         var dx = (pi.level - zxy) / (pi.z[locy][locx + 1] - zxy);
+        // Interpolate, but protect against NaN linear values for log axis (dx will equal 1 or 0)
+        var dxl =
+            (dx !== 1 ? (1 - dx) * xa.c2l(pi.x[locx]) : 0)
+            + (dx !== 0 ? dx * xa.c2l(pi.x[locx + 1]) : 0);
 
-        return [xa.c2p((1 - dx) * pi.x[locx] + dx * pi.x[locx + 1], true),
+        return [xa.l2p(dxl, true),
             ya.c2p(pi.y[locy], true),
             locx + dx, locy];
     } else {
         var dy = (pi.level - zxy) / (pi.z[locy + 1][locx] - zxy);
+        var dyl =
+            (dy !== 1 ? (1 - dy) * ya.c2l(pi.y[locy]) : 0)
+            + (dy !== 0 ? dy * ya.c2l(pi.y[locy + 1]) : 0);
+
         return [xa.c2p(pi.x[locx], true),
-            ya.c2p((1 - dy) * pi.y[locy] + dy * pi.y[locy + 1], true),
+            ya.l2p(dyl, true),
             locx, locy + dy];
     }
 }