The area mark supports [curve options](../features/curves.md) 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://d3js.org/d3-shape/area#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.
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