Merge branch 'chart-performance' into 82-chart-improvements

components/chart/y-axis.js

@@ -0,0 +1,54 @@
+import React from 'react'
+import { Text, View } from 'react-native'
+import config from './config'
+import styles from './styles'
+
+function makeYAxis() {
+  const scaleMin = config.temperatureScale.low
+  const scaleMax = config.temperatureScale.high
+  const numberOfTicks = (scaleMax - scaleMin) * 2
+  const tickDistance = config.chartHeight / numberOfTicks
+
+  const tickPositions = []
+  const labels = []
+  // for style reasons, we don't want the first and last tick
+  for (let i = 1; i < numberOfTicks - 1; i++) {
+    const y = tickDistance * i
+    const style = styles.yAxisLabel
+    // this eyeballing is sadly necessary because RN does not
+    // support percentage values for transforms, which we'd need
+    // to reliably place the label vertically centered to the grid
+    style.top = y - 8
+    labels.push(
+      <Text
+        style={{...style}}
+        key={i}>
+        {scaleMax - i * 0.5}
+      </Text>
+    )
+    tickPositions.push(y)
+  }
+
+  return {labels, tickPositions}
+}
+
+export const yAxis = makeYAxis()
+
+export const horizontalGrid = yAxis.tickPositions.map(tick => {
+  return (
+    <View
+      position='absolute'
+      width='100%'
+      top={tick}
+      {...styles.horizontalGrid}
+      key={tick}
+    />
+  )
+})
+
+export function normalizeToScale(temp) {
+  const scale = config.temperatureScale
+  const valueRelativeToScale = (scale.high - temp) / (scale.high - scale.low)
+  const scaleHeight = config.chartHeight
+  return scaleHeight * valueRelativeToScale
+}