Require 2 uniq points for circularize. Handle 2 point case, where D3 centroid() fails.

js/id/actions/circularize.js

@@ -12,7 +12,7 @@ iD.actions.Circularize = function(wayId, projection, maxAngle) {
             keyNodes = nodes.filter(function(n) { return graph.parentWays(n).length !== 1; }),
             points = nodes.map(function(n) { return projection(n.loc); }),
             keyPoints = keyNodes.map(function(n) { return projection(n.loc); }),
-            centroid = d3.geom.polygon(points).centroid(),
+            centroid = (points.length === 2) ? iD.geo.interp(points[0], points[1], 0.5) : d3.geom.polygon(points).centroid(),
             radius = d3.median(points, function(p) { return iD.geo.euclideanDistance(centroid, p); }),
             sign = d3.geom.polygon(points).area() > 0 ? 1 : -1,
             ids;

js/id/operations/circularize.js

@@ -9,8 +9,10 @@ iD.operations.Circularize = function(selectedIDs, context) {
     };
 
     operation.available = function() {
+        var entity = context.entity(entityId);
         return selectedIDs.length === 1 &&
-            context.entity(entityId).type === 'way';
+            entity.type === 'way' &&
+            _.uniq(entity.nodes).length > 1;
     };
 
     operation.disabled = function() {