Update README

Author: mbostock

README.md

@@ -496,7 +496,7 @@ The **stroke** defaults to none. The **fill** defaults to currentColor if the st
 
 Points along the baseline and topline are connected in input order. Likewise, if there are multiple series via the *z*, *fill*, or *stroke* channel, the series are drawn in input order such that the last series is drawn on top. Typically, the given data is already in sorted order, such as chronological for time series; to sort the data, use a [sort transform](#transforms).
 
-The area mark supports [curve options](#curves) to control interpolation between points. If any of the *x1*, *y1*, *x2*, or *y2* values are invalid (undefined, null, or NaN), the baseline and topline will be interrupted at this point, resulting in a break that separates the series’ shape into multiple segments. (See [d3-shape’s *area*.defined](https://github.com/d3/d3-shape/blob/master/README.md#area_defined) for more.) If an area segment consists of only a single point, it may appear invisible unless rendered with rounded or square line caps. In addition, some curves such as *cardinal-open* only render a visible segment if it contains multiple points.
+The area mark supports [curve options](#curves) to control interpolation between points. If any of the *x1*, *y1*, *x2*, or *y2* values are invalid (undefined, null, or NaN), the baseline and topline will be interrupted, resulting in a break that divides the area shape into multiple segments. (See [d3-shape’s *area*.defined](https://github.com/d3/d3-shape/blob/master/README.md#area_defined) for more.) If an area segment consists of only a single point, it may appear invisible unless rendered with rounded or square line caps. In addition, some curves such as *cardinal-open* only render a visible segment if it contains multiple points.
 
 #### Plot.area(*data*, *options*)
 
@@ -666,13 +666,13 @@ In addition to the [standard mark options](#marks), the following optional chann
 
 * **z** - a categorical value to group data into series
 
-TODO Describe the defaults for fill, stroke, and z. Describe how varying color and opacity within a series is not recommended.
+By default, the data is assumed to represent a single series (a single value that varies over time, *e.g.*). If the **z** channel is specified, data is grouped by *z* to form separate series. Typically *z* is a categorical value such as a series name. If **z** is not specified, it defaults to **stroke** if a channel, or **fill** if a channel.
 
-TODO Describe the importance of data order and the **sort** transform.
+The **stroke** defaults to currentColor. The **fill** defaults to none. If both the stroke and fill are defined as channels, or if the *z* channel is also specified, it is possible for the stroke or fill to vary within a series; varying color within a series is not supported, however, so only the first channel value for each series is considered. This limitation also applies to the **fillOpacity**, **strokeOpacity**, and **title** channels.
 
-TODO Describe how missing or invalid data is handled (broken lines).
+Points along the line are connected in input order. Likewise, if there are multiple series via the *z*, *fill*, or *stroke* channel, the series are drawn in input order such that the last series is drawn on top. Typically, the given data is already in sorted order, such as chronological for time series; to sort the data, use a [sort transform](#transforms).
 
-The line mark supports [curve options](#curves) to control interpolation between points.
+The line mark supports [curve options](#curves) to control interpolation between points. If any of the *x* or *y* values are invalid (undefined, null, or NaN), the line will be interrupted, resulting in a break that divides the line shape into multiple segments. (See [d3-shape’s *line*.defined](https://github.com/d3/d3-shape/blob/master/README.md#line_defined) for more.) If a line segment consists of only a single point, it may appear invisible unless rendered with rounded or square line caps. In addition, some curves such as *cardinal-open* only render a visible segment if it contains multiple points.
 
 #### Plot.line(*data*, *options*)