Axis ticks should optionally draw grid lines about vega HOT 4 CLOSED

Agreed. It turns out you can do this already (kind of) by "hacking" tick mark properties. For example, try adding the following to the Y-axis definition in the Vega bar chart example:

"properties": {
  "ticks": {
    "x": {"value": -5},
    "x2": {"value": 400},
    "stroke": {"value": "#888"},
    "strokeOpacity": {"value": "0.5"}
  }
}

In any case, we'll take more explicit gridline support into consideration for our roadmap.

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This is slightly off-topic, but it'd be useful if scales recognised constants -Inf and Inf, so you could run a line the complete length of a scale without magic constants.

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Tried this hack of extending the ticks into the graph area — they draw on top of other marks! Would it make sense to render axes first/underneath anyway?

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These issues have been addressed in commit 456746a, and will be part of the 1.3.0 release. New features include:

• Axes now support grid lines, though they are disabled by default. To enable grid lines, include the parameter grid: true in your axis definition. To further style the gridlines, include a "grid": {...} entry within the axis properties parameter.
• Axes can now be positioned either above or below data marks. The default behavior is to place an axis above the marks. To place the axes below, add the "layer": "back" parameter to the axis definition ("layer": "front" is also legal, but assumed by default).
• Mark properties, including those of axis elements, can now be set to the full width or height of the current group through a new "group" option for value references (this is similar to @hadley's suggestion of Inf, -Inf extents). Here are some examples: "x2": {"group":"width"} and "y2": {"group":"height"}. Axis gridlines use this feature internally.

Thanks @malecki and @hadley for the suggestions!

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