Fix grid location on manual ticks.

Previously, when manually setting tick positions, the gridlines would
not follow, and instead remain at auto-detection positions, or maybe
some other arbitrary-seeming positions. This fixes it.

Disclaimer: this was co-developed with gpt-5.1-codex, which identified
the issue and proposed the fix, I tested and simplified its code a bit.

Author: lucasb-eyer

src/core/axis.js

@@ -82,10 +82,11 @@ function mpld3_Axis(ax, props) {
 }
 
 mpld3_Axis.prototype.getGrid = function() {
+    this.filter_ticks(this.scale.domain());
     var gridprop = {
         nticks: this.props.nticks,
         zorder: this.props.zorder,
-        tickvalues: null,
+        tickvalues: this.props.filtered_tickvalues ?? this.props.tickvalues,
         xy: this.props.xy
     }
     if (this.props.grid) {