feat: Add basic Stacked charts support (#22)

library/src/main/java/com/dzeio/charts/ChartView.kt

@@ -144,8 +144,14 @@ class ChartView @JvmOverloads constructor(context: Context?, attrs: AttributeSet
             )
         }
 
-        for (serie in series) {
+        if (type == ChartType.STACKED) {
-            serie.onDraw(canvas, rect)
+            for (serie in series.reversed()) {
+                serie.onDraw(canvas, rect)
+            }
+        } else {
+            for (serie in series) {
+                serie.onDraw(canvas, rect)
+            }
         }
         super.onDraw(canvas)
     }

library/src/main/java/com/dzeio/charts/axis/YAxis.kt

@@ -5,9 +5,12 @@ import android.graphics.Color
 import android.graphics.Paint
 import android.graphics.Rect
 import android.graphics.RectF
+import com.dzeio.charts.ChartType
 import com.dzeio.charts.ChartViewInterface
+import com.dzeio.charts.Entry
 import com.dzeio.charts.utils.drawDottedLine
 import kotlin.math.roundToInt
+import kotlin.math.sign
 
 class YAxis(
     private val view: ChartViewInterface
@@ -37,7 +40,7 @@ class YAxis(
 
     override var labelCount = 5
 
-    private var min: Float? = 0f
+    private var min: Float? = null
     private var max: Float? = null
 
     @Deprecated("use the new global function", replaceWith = ReplaceWith("YAxisInterface.addLine"))
@@ -68,6 +71,22 @@ class YAxis(
         if (view.series.isEmpty()) {
             return this.lines.keys.maxOrNull() ?: 0f
         }
+        if (view.type == ChartType.STACKED) {
+            val nList: ArrayList<Float> = arrayListOf()
+
+            for (serie in view.series) {
+                val size = serie.entries.size
+                while (nList.size < size) {
+                    nList.add(0f)
+                }
+                for (index in 0 until serie.entries.size) {
+                    val entry = serie.entries[index]
+                    nList[index] += entry.y
+                }
+            }
+
+            return nList.maxOf { it }
+        }
         val seriesMax = view.series
             .maxOf { serie ->
                 if (serie.getDisplayedEntries().isEmpty()) {
@@ -85,6 +104,22 @@ class YAxis(
         if (view.series.isEmpty()) {
             return this.lines.keys.minOrNull() ?: 0f
         }
+        if (view.type == ChartType.STACKED) {
+            val nList: ArrayList<Float> = arrayListOf()
+
+            for (serie in view.series) {
+                val size = serie.entries.size
+                while (nList.size < size) {
+                    nList.add(0f)
+                }
+                for (index in 0 until serie.entries.size) {
+                    val entry = serie.entries[index]
+                    nList[index] += entry.y
+                }
+            }
+
+            return nList.minOf { it }
+        }
         return view.series
             .minOf { serie ->
                 if (serie.getDisplayedEntries().isEmpty()) {
@@ -168,4 +203,35 @@ class YAxis(
             addLine(height, Line(true))
         }
     }
+
+    override fun getPositionOnRect(entry: Entry, drawableSpace: RectF): Float {
+        if (view.type == ChartType.STACKED) {
+            val serie = view.series.find { it.entries.contains(entry) }
+            val index = view.series.indexOf(serie)
+            return getPositionOnRect(entry, drawableSpace, index)
+        }
+        return getPositionOnRect(entry.y, drawableSpace)
+    }
+
+    private fun getPositionOnRect(entry: Entry, drawableSpace: RectF, index: Int): Float {
+        if (index > 0) {
+            val entry2 = view.series[index - 1].entries.find { it.x == entry.x }
+            if (entry2 != null) {
+                // make a new """Entry""" containing the new Y
+                val isReverse = sign(entry2.y) != sign(entry.y)
+                val tmp = Entry(entry.x, if (isReverse) entry.y else entry.y + entry2.y)
+                return getPositionOnRect(tmp, drawableSpace, index - 1)
+            }
+        }
+        return getPositionOnRect(entry.y, drawableSpace)
+    }
+
+    override fun getPositionOnRect(point: Float, drawableSpace: RectF): Float {
+        val min = getYMin()
+        val max = getYMax()
+
+        return (1 - (point - min) / (max - min)) *
+            drawableSpace.height() +
+            drawableSpace.top
+    }
 }

library/src/main/java/com/dzeio/charts/axis/YAxisInterface.kt

@@ -3,6 +3,7 @@ package com.dzeio.charts.axis
 import android.graphics.Canvas
 import android.graphics.Paint
 import android.graphics.RectF
+import com.dzeio.charts.Entry
 
 sealed interface YAxisInterface {
 
@@ -110,4 +111,24 @@ sealed interface YAxisInterface {
      * @param y the Y position of the line
      */
     fun removeLine(y: Float)
+
+    /**
+     * get the position of an [entry] Y position in the [drawableSpace]
+     *
+     * if the chart type is stacked it will automatically calculate the position depending on it
+     *
+     * @param entry the entry to search to position
+     * @param drawableSpace the space in which it should appear
+     * @return the float position (can be out of the [drawableSpace])
+     */
+    fun getPositionOnRect(entry: Entry, drawableSpace: RectF): Float
+
+    /**
+     * get the position of a [point] in the [drawableSpace]
+     *
+     * @param point the point to search to position
+     * @param drawableSpace the space in which it should appear
+     * @return the float position (can be out of the [drawableSpace])
+     */
+    fun getPositionOnRect(point: Float, drawableSpace: RectF): Float
 }

library/src/main/java/com/dzeio/charts/series/BarSerie.kt

@@ -44,17 +44,12 @@ class BarSerie(
     override fun onDraw(canvas: Canvas, drawableSpace: RectF) {
         val displayedEntries = getDisplayedEntries()
         val barWidth = view.xAxis.getEntryWidth(drawableSpace).toFloat()
-        val max = view.yAxis.getYMax()
-        val min = view.yAxis.getYMin()
 
-        val zero = ((1 - -min / (max - min)) * drawableSpace.height() + drawableSpace.top).coerceIn(
+        val zero = view.yAxis.getPositionOnRect(0f, drawableSpace)
-            drawableSpace.top, drawableSpace.bottom
-        )
 
         for (entry in displayedEntries) {
             // calculated height in percent from 0 to 100
-            val top = ((1 - (entry.y - min) / (max - min)) * drawableSpace.height() + drawableSpace.top)
+            val top = view.yAxis.getPositionOnRect(entry, drawableSpace)
-                .coerceIn(drawableSpace.top, drawableSpace.bottom)
             var posX = drawableSpace.left + view.xAxis.getPositionOnRect(
                 entry,
                 drawableSpace

library/src/main/java/com/dzeio/charts/series/LineSerie.kt

@@ -34,15 +34,13 @@ class LineSerie(
     override fun onDraw(canvas: Canvas, drawableSpace: RectF) {
         val displayedEntries = getDisplayedEntries()
         displayedEntries.sortBy { it.x }
-        val max = view.yAxis.getYMax()
-        val min = view.yAxis.getYMin()
 
         var previousPosX: Float? = null
         var previousPosY: Float? = null
 
         for (entry in displayedEntries) {
             // calculated height in percent from 0 to 100
-            val top = (1 - (entry.y - min) / (max - min)) * drawableSpace.height() + drawableSpace.top
+            val top = view.yAxis.getPositionOnRect(entry, drawableSpace)
             val posX = (drawableSpace.left +
                     view.xAxis.getPositionOnRect(entry, drawableSpace) +
                     view.xAxis.getEntryWidth(drawableSpace) / 2f).toFloat()

sample/src/main/java/com/dzeio/chartstest/ui/MainFragment.kt

@@ -37,15 +37,15 @@ class MainFragment : Fragment() {
             val serie2 = BarSerie(this)
 
             // transform the chart into a grouped chart
-            type = ChartType.GROUPED
+            type = ChartType.STACKED
 
             // utils function to use Material3 auto colors
             materielTheme(this, requireView())
             serie2.barPaint.color = Color.RED
 
             // give the serie it's entries
-            serie1.entries = generateRandomDataset(10)
+            serie1.entries = generateRandomDataset(1)
-            serie2.entries = generateRandomDataset(10)
+            serie2.entries = generateRandomDataset(1)
 
             // refresh the Chart
             refresh()
@@ -164,7 +164,7 @@ class MainFragment : Fragment() {
     private fun generateRandomDataset(size: Int = 100, min: Int = 0, max: Int = 100): ArrayList<Entry> {
         val dataset: ArrayList<Entry> = arrayListOf()
 
-        for (i in 0 .. size) {
+        for (i in 0 until size) {
             dataset.add(Entry(
                 i.toDouble(),
                 Random.nextInt(min, max).toFloat()