* make link work in github itself
* fix dead link on reload or new tab
If the callback returns `null` or `undefined` the associated grid line will be hidden.
:::tip
-The [category axis](../axes/cartesian/category), which is the default x-axis for line and bar charts, uses the `index` as internal data format. For accessing the label, use `this.getLabelForValue(value)`. [API: getLabelForValue](../api/classes/scale.html#getlabelforvalue)
+The [category axis](../axes/cartesian/category), which is the default x-axis for line and bar charts, uses the `index` as internal data format. For accessing the label, use `this.getLabelForValue(value)`. [API: getLabelForValue](../api/classes/Scale.html#getlabelforvalue)
:::
In the following example, every label of the Y-axis would be displayed with a dollar sign at the front.