feat: fill line plot data with undefined

src/transforms.spec.ts

@@ -3,14 +3,20 @@ import type { Datapoint } from './transforms'
 
 describe('module transforms', () => {
 	describe('transformToLinePlotData', () => {
-		it('returns empty datapoints on empty series', () => {
+		it('returns y undefined on empty series', () => {
 			const input: Datapoint<number>[] = []
 			const output = transformToLinePlotData(input, {
 				startAt: 100,
 				endAt: 130,
 				stepSize: 10,
 			})
-			expect(output).toHaveLength(0)
+			const expectedDatapoints = [
+				{ x: 100, y: undefined },
+				{ x: 110, y: undefined },
+				{ x: 120, y: undefined },
+				{ x: 130, y: undefined },
+			]
+			expect(output).toEqual(expectedDatapoints)
 		})
 
 		it('returns empty datapoints if startAt > endAt', () => {
@@ -26,7 +32,7 @@ describe('module transforms', () => {
 			expect(output).toHaveLength(0)
 		})
 
-		it('returns empty datapoints on input after range', () => {
+		it('returns y undefined on input after range', () => {
 			const input: Datapoint<number>[] = [
 				{ x: 200, y: 1 },
 				{ x: 300, y: 2 },
@@ -36,7 +42,13 @@ describe('module transforms', () => {
 				endAt: 130,
 				stepSize: 10,
 			})
-			expect(output).toHaveLength(0)
+			const expectedDatapoints = [
+				{ x: 100, y: undefined },
+				{ x: 110, y: undefined },
+				{ x: 120, y: undefined },
+				{ x: 130, y: undefined },
+			]
+			expect(output).toEqual(expectedDatapoints)
 		})
 
 		it('repeats last datapoint on input before range', () => {
@@ -93,7 +105,7 @@ describe('module transforms', () => {
 				stepSize: 10,
 			})
 			const expectedDatapoints = [
-				// no data point for x = 100
+				{ x: 100, y: undefined },
 				{ x: 110, y: 1 },
 				{ x: 120, y: 4 },
 				{ x: 130, y: 4 },

src/transforms.ts

@@ -8,26 +8,40 @@ export type Datapoint<Y> = { x: number; y: Y }
  * and then creating a new data series by tracing the line and noting
  * an (x/y) pair at fixed intervals (steps of x).
  *
+ * Please note:
+ * If there is no data (at the beginning), data points will be created
+ * for it with `y = undefined`.
+ * As a consequence, the size of the resulting array depends only on
+ * the options, not on the input data.
+ *
  * @param datapoints The data points to be transformed
  * @param options Controls the transformation
  * @returns The transformed data points
  */
 export function transformToLinePlotData<Y>(
 	datapoints: Datapoint<Y>[],
 	options: { stepSize: number; startAt: number; endAt: number }
-): Datapoint<Y>[] {
-	const result: Datapoint<Y>[] = []
-	if (datapoints.length === 0) return result
-
+): Datapoint<Y | undefined>[] {
+	const result: Datapoint<Y | undefined>[] = []
 	const { startAt, endAt, stepSize } = options
 
+	if (datapoints.length === 0) {
+		for (let x = startAt; x <= endAt; x += stepSize) {
+			result.push({ x, y: undefined })
+		}
+		return result
+	}
+
 	// indexes into series.datapoints
 	let lastIdx = 0
 	let nextIdx
 	let y = datapoints[lastIdx].y
 	let endReached = false
 	for (let x = startAt; x <= endAt; x += stepSize) {
-		if (datapoints[lastIdx].x > x) continue
+		if (datapoints[lastIdx].x > x) {
+			result.push({ x, y: undefined })
+			continue
+		}
 		if (!endReached) {
 			// move forward, until we cross x or we reach the end
 			nextIdx = lastIdx + 1