Update README

Author: mbostock

README.md

@@ -490,11 +490,11 @@ In addition to the [standard mark options](#marks), the following optional chann
 
 If **x2** is not specified, it defaults to **x1**. If **y2** is not specified, it defaults to **y1**. These defaults facilitate sharing *x* or *y* coordinates between the baseline and topline. See also the implicit stack transform and shorthand **x** and **y** options supported by [Plot.areaY](#plotareaydata-options) and [Plot.areaX](#plotareaxdata-options).
 
-By default, the data is assumed to represent a single series (*e.g.*, a single value over time). 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 **fill** if a channel, or **stroke** if a channel.
+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 **fill** if a channel, or **stroke** if a channel.
 
-The **stroke** color defaults to none. The **fill** color defaults to currentColor if the stroke is none, and to none otherwise. If both the stroke and fill are defined as channels, or if the *z* channel is also specified, it is possible that the stroke or fill could vary within a series. Varying color within a series is not supported, so only the first value for each series is considered. This limitation also applies to the **fillOpacity**, **strokeOpacity**, and **title** channels.
+The **stroke** defaults to none. The **fill** defaults to currentColor if the stroke is none, and to none otherwise. 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.
 
-Points along the baseline and topline are connected in input order. Likewise, if there are multiple series via the *z*, *fill*, or *stroke* channel, 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).
+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.