diff --git a/packages/polaris-viz-core/src/types.ts b/packages/polaris-viz-core/src/types.ts
index 959004c6f8..508deb79da 100644
--- a/packages/polaris-viz-core/src/types.ts
+++ b/packages/polaris-viz-core/src/types.ts
@@ -15,6 +15,9 @@ export interface DataSeries {
   name?: string;
   metadata?: {[key: string]: any};
   styleOverride?: StyleOverride;
+  /**
+  * Value that gets used to fill in missing data points. Defaults to `null`.
+  */
   fillValue?: DataPoint['value'];
 }