relative barmode to stack negative values below axis

src/traces/bar/layout_attributes.js

@@ -12,13 +12,15 @@
 module.exports = {
     barmode: {
         valType: 'enumerated',
-        values: ['stack', 'group', 'overlay'],
+        values: ['stack', 'group', 'overlay', 'relative'],
         dflt: 'group',
         role: 'info',
         description: [
             'Determines how bars at the same location coordinate',
             'are displayed on the graph.',
             'With *stack*, the bars are stacked on top of one another',
+            'With *relative*, the bars are stacked on top of one another,',
+            'with negative values below the axis, positive values above',
             'With *group*, the bars are plotted next to one another',
             'centered around the shared location.',
             'With *overlay*, the bars are plotted over one another,',

src/traces/bar/set_positions.js

@@ -116,10 +116,11 @@ module.exports = function setPositions(gd, plotinfo) {
         else barposition(bl);
 
         var stack = (fullLayout.barmode === 'stack'),
+            relative = (fullLayout.barmode ==='relative'),
             norm = fullLayout.barnorm;
 
         // bar size range and stacking calculation
-        if(stack || norm) {
+        if(stack || relative || norm) {
             // for stacked bars, we need to evaluate every step in every
             // stack, because negative bars mean the extremes could be
             // anywhere
@@ -142,13 +143,15 @@ module.exports = function setPositions(gd, plotinfo) {
                 ti = gd.calcdata[bl[i]];
                 for(j = 0; j < ti.length; j++) {
                     sv = Math.round(ti[j].p / sumround);
+                    // store the negative sum value for p at the same key, with sign flipped
+                    if(relative && ti[j].s < 0) sv = -sv;
                     var previousSum = sums[sv] || 0;
-                    if(stack) ti[j].b = previousSum;
+                    if(stack || relative) ti[j].b = previousSum;
                     barEnd = ti[j].b + ti[j].s;
                     sums[sv] = previousSum + ti[j].s;
 
                     // store the bar top in each calcdata item
-                    if(stack) {
+                    if(stack || relative) {
                         ti[j][sLetter] = barEnd;
                         if(!norm && isNumeric(sa.c2l(barEnd))) {
                             sMax = Math.max(sMax,barEnd);
@@ -161,13 +164,18 @@ module.exports = function setPositions(gd, plotinfo) {
             if(norm) {
                 padded = false;
                 var top = norm==='fraction' ? 1 : 100,
+                    relAndNegative = false,
                     tiny = top/1e9; // in case of rounding error in sum
                 sMin = 0;
                 sMax = stack ? top : 0;
                 for(i = 0; i < bl.length; i++) { // trace index
                     ti = gd.calcdata[bl[i]];
                     for(j = 0; j < ti.length; j++) {
-                        scale = top / sums[Math.round(ti[j].p/sumround)];
+                        relAndNegative = relative && ti[j].s < 0;
+                        sv = Math.round(ti[j].p / sumround);
+                        if(relAndNegative) sv = -sv;  // locate negative sum amount for this p val
+                        scale = top / sums[sv];
+                        if(relAndNegative) scale *= -1; // preserve sign if negative
                         ti[j].b *= scale;
                         ti[j].s *= scale;
                         barEnd = ti[j].b + ti[j].s;

test/image/mocks/bar_stackrelative_negative.json

@@ -0,0 +1,33 @@
+{
+    "data":[
+        {
+            "name":"Col1",
+            "y":["-1","2","3","4","5"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        },
+        {
+            "name":"Col2",
+            "y":["2","3","4","-3","2"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        },
+        {
+            "name":"Col3",
+            "y":["5","4","3","-2","1"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        },
+        {
+            "name":"Col4",
+            "y":["-3","0","1","0","-3"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        }
+    ],
+    "layout":{
+        "height":400,
+        "width":400,
+        "barmode":"relative"
+    }
+}

test/image/mocks/bar_stackrelativeto100_negative.json

@@ -0,0 +1,34 @@
+{
+    "data":[
+        {
+            "name":"Col1",
+            "y":["-1","2","3","4","5"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        },
+        {
+            "name":"Col2",
+            "y":["2","3","4","-3","2"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        },
+        {
+            "name":"Col3",
+            "y":["5","4","3","-2","1"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        },
+        {
+            "name":"Col4",
+            "y":["-3","0","1","0","-3"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        }
+    ],
+    "layout":{
+        "height":400,
+        "width":400,
+        "barmode":"relative",
+        "barnorm":"percent"
+    }
+}

test/image/mocks/bar_stackto100_negative.json

@@ -1,9 +1,29 @@
 {
     "data":[
-        {"name":"Col1","y":["1","2","3","4","5"],"x":["1","2","3","4","5"],"type":"bar","uid":"aeb9ea"},
-        {"name":"Col2","y":["2","3","4","3","2"],"x":["1","2","3","4","5"],"type":"bar","uid":"2f201d"},
-        {"name":"Col3","y":["5","4","3","2","1"],"x":["1","2","3","4","5"],"type":"bar","uid":"aef0bf"},
-        {"name":"Col4","y":["-1","0","1","0","-1"],"x":["1","2","3","4","5"],"type":"bar","uid":"330b4d"}
+        {
+            "name":"Col1",
+            "y":["1","2","3","4","5"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        },
+        {
+            "name":"Col2",
+            "y":["2","3","4","3","2"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        },
+        {
+            "name":"Col3",
+            "y":["5","4","3","2","1"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        },
+        {
+            "name":"Col4",
+            "y":["-1","0","1","0","-1"],
+            "x":["1","2","3","4","5"],
+            "type":"bar"
+        }
     ],
     "layout":{
         "height":400,