Rule

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// Single View Specification
{
  "data": ... ,
  "mark": "rule",
  "encoding": ... ,
  ...
}

The rule mark represents each data point as a line segment. It can be used in two ways. First, as a line segment that spans the complete width or height of a view. Second, a rule can be used to draw a line segment between two positions.

Documentation Overview

Width/Height-Spanning Rules

If the rule mark only has y encoding, the output view produces horizontal rules that spans the complete width. Similarly, if the rule mark only has x encoding, the output view produces vertical rules that spans the height.

We can use rules to show the average price of different stocks akin to tick marks.

The fact that rule marks span the width or the height of a single view make them useful as an annotation layer. For example, we can use rules to show average values of different stocks alongside the price curve.

We can also use a rule mark to show global mean value over a histogram.

Ranged Rules

To control the spans of horizontal/vertical rules, x and x2/y and y2 channels can be specified.

For example, we can use y and y2 show the "min" and "max" values of horsepowers for cars from different locations.

We can also use rule to create error bars. In the example below, we use the ci0 and ci1 aggregation operators to visualize the 95% confidence interval as a measure of the spread of the average yields for a variety of barley strains.

Alternatively, we can create error bars showing one standard deviation (stdev) over and below the mean value.

Rule Config

// Top-level View Specification
{
  ...
  "config": {
    "rule": ...,
    ...
  }
}

The rule property of the top-level config object sets the default properties for all rule marks. If mark property encoding channels are specified for marks, these config values will be overridden.

For the list of all supported properties, please see the mark config documentation.