From d8b612b5fb4ff22f9249d4a274ab985c7be7716d Mon Sep 17 00:00:00 2001
From: Dan Vanderkam <danvdk@gmail.com>
Date: Mon, 1 Jul 2013 23:42:31 -0400
Subject: [PATCH] interpolation

---
 extras/hairlines.js  | 90 +++++++++++++++++++++++++++++++++++++++++-----------
 tests/hairlines.html | 24 ++++++++++++++
 2 files changed, 96 insertions(+), 18 deletions(-)

diff --git a/extras/hairlines.js b/extras/hairlines.js
index 92bbc25..8e3d7dd 100644
--- a/extras/hairlines.js
+++ b/extras/hairlines.js
@@ -192,6 +192,33 @@ hairlines.prototype.updateHairlineStyles = function() {
   });
 };
 
+// Find prevRow and nextRow such that
+// g.getValue(prevRow, 0) <= xval
+// g.getValue(nextRow, 0) >= xval
+// g.getValue({prev,next}Row, col) != null, NaN or undefined
+// and there's no other row such that:
+//   g.getValue(prevRow, 0) < g.getValue(row, 0) < g.getValue(nextRow, 0)
+//   g.getValue(row, col) != null, NaN or undefined.
+// Returns [prevRow, nextRow]. Either can be null (but not both).
+hairlines.findPrevNextRows = function(g, xval, col) {
+  var prevRow = null, nextRow = null;
+  var numRows = g.numRows();
+  for (var row = 0; row < numRows; row++) {
+    var yval = g.getValue(row, col);
+    if (yval === null || yval === undefined || isNaN(yval)) continue;
+
+    var rowXval = g.getValue(row, 0);
+    if (rowXval <= xval) prevRow = row;
+
+    if (rowXval >= xval) {
+      nextRow = row;
+      break;
+    }
+  }
+
+  return [prevRow, nextRow];
+};
+
 // Fills out the info div based on current coordinates.
 hairlines.prototype.updateHairlineInfo = function() {
   var mode = 'closest';
@@ -200,31 +227,58 @@ hairlines.prototype.updateHairlineInfo = function() {
   var xRange = g.xAxisRange();
   var that = this;
   $.each(this.hairlines_, function(idx, h) {
-    var row = null;
-    if (mode == 'closest') {
-      // TODO(danvk): make this dygraphs method public
-      row = g.findClosestRow(g.toDomXCoord(h.xval));
-    } else if (mode == 'interpolate') {
-      // ...
-    }
-
-    // To use generateLegendHTML, we have to synthesize an array of selected
-    // points.
+    // To use generateLegendHTML, we synthesize an array of selected points.
     var selPoints = [];
     var labels = g.getLabels();
-    for (var i = 1; i < g.numColumns(); i++) {
-      selPoints.push({
-        canvasx: 1,
-        canvasy: 1,
-        xval: h.xval,
-        yval: g.getValue(row, i),
-        name: labels[i]
-      });
+    var row, prevRow, nextRow;
+
+    if (!h.interpolated) {
+      // "closest point" mode.
+      // TODO(danvk): make findClosestRow method public
+      row = g.findClosestRow(g.toDomXCoord(h.xval));
+      for (var i = 1; i < g.numColumns(); i++) {
+        selPoints.push({
+          canvasx: 1,  // TODO(danvk): real coordinate
+          canvasy: 1,  // TODO(danvk): real coordinate
+          xval: h.xval,
+          yval: g.getValue(row, i),
+          name: labels[i]
+        });
+      }
+    } else {
+      // "interpolated" mode.
+      for (var i = 1; i < g.numColumns(); i++) {
+        var prevNextRow = hairlines.findPrevNextRows(g, h.xval, i);
+        prevRow = prevNextRow[0], nextRow = prevNextRow[1];
+
+        // For x-values outside the domain, interpolate "between" the extreme
+        // point and itself.
+        if (prevRow === null) prevRow = nextRow;
+        if (nextRow === null) nextRow = prevRow;
+
+        // linear interpolation
+        var prevX = g.getValue(prevRow, 0),
+            nextX = g.getValue(nextRow, 0),
+            prevY = g.getValue(prevRow, i),
+            nextY = g.getValue(nextRow, i),
+            frac = prevRow == nextRow ? 0 : (h.xval - prevX) / (nextX - prevX),
+            yval = frac * nextY + (1 - frac) * prevY;
+
+        selPoints.push({
+          canvasx: 1,  // TODO(danvk): real coordinate
+          canvasy: 1,  // TODO(danvk): real coordinate
+          xval: h.xval,
+          yval: yval,
+          name: labels[i]
+        });
+      }
     }
 
     if (that.divFiller_) {
       that.divFiller_(h.infoDiv, {
         closestRow: row,
+        prevRow: prevRow,
+        nextRow: nextRow,
         points: selPoints,
         hairline: that.createPublicHairline_(h),
         dygraph: g
diff --git a/tests/hairlines.html b/tests/hairlines.html
index fcdf3f8..e92ba96 100644
--- a/tests/hairlines.html
+++ b/tests/hairlines.html
@@ -145,6 +145,12 @@
       <button id="add-button">Add a Hairline</button>
       <button id="remove-button">Remove a Hairline</button>
       <button id="reset-button">Reset Hairlines</button>
+      <br/>
+      Hairline mode:
+      <input type=radio name="hairline-mode" id="hairline-interpolated" checked=true>
+      <label for="hairline-interpolated"> Interpolated</label>
+      <input type=radio name="hairline-mode" id="hairline-closest">
+      <label for="hairline-closest"> Closest</label>
     </div>
 
     <script type="text/javascript">
@@ -231,14 +237,32 @@
       $('#reset-button').on('click', function(e) {
         setDefaultState();
       });
+      function setHairlineModeRadio() {
+        var hs = hairlines.get();
+        if (hs.length) {
+          var interpolated = hs[0].interpolated;
+          $('#hairline-interpolated').prop('checked', interpolated);
+          $('#hairline-closest').prop('checked', !interpolated);
+        }
+      }
+      $('[name=hairline-mode]').change(function() {
+        var interpolated = $('#hairline-interpolated').is(':checked');
+        var hs = hairlines.get();
+        for (var i = 0; i < hs.length; i++) {
+          hs[i].interpolated = interpolated;
+        }
+        hairlines.set(hs);
+      });
 
       // Persistence
       function loadFromStorage() {
         hairlines.set(JSON.parse(localStorage.getItem('hairlines')));
         annotations.set(JSON.parse(localStorage.getItem('annotations')));
+        setHairlineModeRadio();
       }
       $(hairlines).on('hairlinesChanged', function(e) {
         localStorage.setItem('hairlines', JSON.stringify(hairlines.get()));
+        setHairlineModeRadio();
       });
       $(annotations).on('annotationsChanged', function(e) {
         localStorage.setItem('annotations', JSON.stringify(annotations.get()));
-- 
2.7.4