Invert sign of scroll step

Author: blink1073

lib/matplotlib/backends/web_backend/mpl.js

@@ -149,7 +149,7 @@ mpl.figure.prototype._init_canvas = function() {
     canvas_div.on("wheel mousewheel", function (event) {
         event = event.originalEvent;
         event['data'] = 'scroll'
-        event.step = event.deltaY / 3.
+        event.step = -event.deltaY / 3.
         mouse_event_fn(event);
     });