Histogram (#888)

* Add Histogram as a visualization

The css and js file use the histogram code from https://bl.ocks.org/mbostock/3048450.
THe viz.py extends from BaseViz to create chart data only for one histogram

* using d3.layout.histogram

* CSS updated

The new css has been used from the d3 chart http://bl.ocks.org/mbostock/1933560

* bars are visible

* added semicolons

* histogram from http://bl.ocks.org/mbostock/1933560

It takes as input no of bins. The histogram cycles through
a set of colors for different lengths of the bar. It places a
y axis coordinate on top or on the upper end of the bar
whichever is suitable.

* update style changes

caravel/assets/javascripts/modules/caravel.js

@@ -32,6 +32,7 @@ const sourceMap = {
   cal_heatmap: 'cal_heatmap.js',
   horizon: 'horizon.js',
   mapbox: 'mapbox.jsx',
+  histogram: 'histogram.js',
 };
 const color = function () {
   // Color related utility functions go in this object

caravel/assets/visualizations/histogram.css

@@ -0,0 +1,17 @@
+.axis line {
+  fill: none;
+  stroke: black;
+  shape-rendering: crispEdges;
+}
+
+.axis text {
+  font-family: "Roboto", "Helvetica Neue", Helvetica, Arial, sans-serif;
+  font-weight: bold;
+  font-size: 15px;  
+}
+
+.axis path, .axis line {
+    fill: none;
+    stroke: #000;
+    shape-rendering: crispEdges;
+}

caravel/assets/visualizations/histogram.js

@@ -0,0 +1,152 @@
+// JS
+const d3 = require('d3')
+const px = window.px || require('../javascripts/modules/caravel.js')
+
+// CSS
+require('./histogram.css')
+
+function histogram(slice) {
+
+    const div = d3.select(slice.selector)
+
+    const _draw = function(data, numBins) {
+
+        // Set Margins
+        const margin = { 
+            top: 50,
+            right: 10,
+            bottom: 20,
+            left: 50,
+        };
+        const navBarHeight = 36;
+        const navBarTitleSize = 12;
+        const navBarBuffer = 10;
+        const width = slice.width() - margin.left - margin.right;
+        const height = slice.height() - margin.top - margin.bottom - navBarHeight - navBarBuffer;
+
+        // Set Histogram objects
+        const formatNumber = d3.format(',.0f');
+        const formatTicks = d3.format(',.00f');
+        const x = d3.scale.ordinal();
+        const y = d3.scale.linear();
+        const xAxis = d3.svg.axis().scale(x).orient('bottom').ticks(numBins).tickFormat(formatTicks);
+        const yAxis = d3.svg.axis().scale(y).orient('left').ticks(numBins*3);
+        // Calculate bins for the data
+        const bins = d3.layout.histogram().bins(numBins)(data);
+
+        // Set the x-values 
+        x.domain(bins.map(function(d) { return d.x;}))
+         .rangeRoundBands([0, width], .1); 
+        // Set the y-values
+        y.domain([0, d3.max(bins, function(d) { return d.y;})])
+         .range([height, 0]);
+
+        // Create the svg value with the bins
+        const svg = div.selectAll('svg').data([bins]).enter().append('svg');
+
+        // Make a rectangular background fill
+        svg.append('rect')
+           .attr('width', '100%')
+           .attr('height', '100%')
+           .attr('fill', '#f6f6f6');
+
+        // Transform the svg to make space for the margins
+        const gEnter = svg
+            .append('g')
+            .attr('transform', 'translate(' + margin.left + ',' + margin.top + ')');
+
+        // Add the bars and the x axis
+        gEnter.append('g').attr('class', 'bars');
+        gEnter.append('g').attr('class', 'x axis');
+
+        // Add width and height to the svg
+        svg.attr('width', slice.width())
+           .attr('height', slice.height());
+
+       // Create the bars in the svg
+        const bar = svg.select('.bars').selectAll('.bar').data(bins);
+        bar.enter().append('rect');
+        bar.exit().remove();
+        // Set the Height and Width for each bar 
+        bar .attr('width', x.rangeBand())
+            .attr('x', function(d) { return x(d.x); })
+            .attr('y', function(d) { return y(d.y); })
+            .attr('height', function(d) {
+                return y.range()[0] - y(d.y);
+            })
+            .attr('fill', function(d) { return px.color.category21(d.length); })
+            .order();
+
+       // Find maximum length to position the ticks on top of the bar correctly 
+       const maxLength = d3.max(bins, function(d) { return d.length;});  
+       function textAboveBar(d) {
+         return d.length/maxLength < 0.1;
+       }
+
+       // Add a bar text to each bar in the histogram 
+       svg.selectAll('.bartext')
+          .data(bins)
+          .enter()
+          .append('text')
+          .attr('dy', '.75em')
+          .attr('y', function(d) { 
+            let padding = 0.0
+            if (textAboveBar(d)) {
+              padding = 12.0
+            } else {
+              padding = -8.0
+            }
+            return y(d.y) - padding;
+          })
+          .attr('x', function(d) { return x(d.x) + (x.rangeBand()/2);})
+          .attr('text-anchor', 'middle')
+          .attr('font-weight', 'bold')
+          .attr('font-size', '15px')
+          .text(function(d) { return formatNumber(d.y); })
+          .attr('fill', function(d) {
+            if(textAboveBar(d)) { return 'black'; } else { return 'white'; }
+          })
+          .attr('transform', 'translate(' + margin.left + ',' + margin.top + ')');
+
+        // Update the x-axis 
+        svg.append('g')
+           .attr('class', 'axis')
+           .attr('transform', 'translate(' + margin.left + ',' + (height + margin.top) + ')')
+           .text('values')
+           .call(xAxis);
+
+        // Update the Y Axis and add minor lines
+        svg.append('g')
+            .attr('class', 'axis')
+            .attr('transform', 'translate(' + margin.left + ',' + margin.top + ')')
+            .text('count')
+            .call(yAxis)
+            .selectAll('g')
+            .filter(function(d) { return d; })
+            .classed('minor', true);
+    };
+
+    const render = function() {
+
+        d3.json(slice.jsonEndpoint(), function(error, json) {
+            if(error !== null) {
+                slice.error(error.responseText, error);
+                return '';
+            }
+
+            const numBins = Number(json.form_data.link_length) || 10;
+
+            div.selectAll('*').remove();
+            _draw(json.data, numBins);
+            slice.done(json);
+        });
+    };
+
+   return {
+        render: render,
+        resize: render,
+   };
+}
+
+module.exports = histogram;
+

caravel/viz.py

@@ -1201,6 +1201,74 @@ def get_data(self):
         return df.to_dict(orient="records")
 
 
+class HistogramViz(BaseViz):
+
+    """Histogram"""
+
+    viz_type = "histogram"
+    verbose_name = _("Histogram")
+    is_timeseries = False
+    fieldsets = ({
+        'label': None,
+        'fields': (
+            ('all_columns_x',),
+            'row_limit',
+        )
+    }, {
+        'label': _("Histogram Options"),
+        'fields': (
+            'link_length',
+        )
+    },)
+
+    form_overrides = {
+        'all_columns_x': {
+            'label': _('Numeric Column'),
+            'description': _("Select the numeric column to draw the histogram"),
+        },
+        'link_length': {
+            'label': _("No of Bins"),
+            'description': _("Select number of bins for the histogram"),
+            'default': 5
+        }
+    }
+
+
+    def query_obj(self):
+        """Returns the query object for this visualization"""
+        d = super(HistogramViz, self).query_obj()
+        d['row_limit'] = self.form_data.get('row_limit', int(config.get('ROW_LIMIT')))
+        numeric_column = self.form_data.get('all_columns_x')
+        if numeric_column is None:
+            raise Exception("Must have one numeric column specified")
+        d['columns'] = [numeric_column]
+        return d
+
+
+    def get_df(self, query_obj=None):
+        """Returns a pandas dataframe based on the query object"""
+        if not query_obj:
+            query_obj = self.query_obj()
+
+        self.results = self.datasource.query(**query_obj)
+        self.query = self.results.query
+        df = self.results.df
+
+        if df is None or df.empty:
+            raise Exception("No data, to build histogram")
+
+        df.replace([np.inf, -np.inf], np.nan)
+        df = df.fillna(0)
+        return df
+
+
+    def get_data(self):
+        """Returns the chart data"""
+        df = self.get_df()
+        chart_data = df[df.columns[0]].values.tolist()
+        return chart_data
+
+
 class DistributionBarViz(DistributionPieViz):
 
     """A good old bar chart"""
@@ -1921,6 +1989,7 @@ def get_data(self):
     CalHeatmapViz,
     HorizonViz,
     MapboxViz,
+    HistogramViz,
     SeparatorViz,
 ]
 

docs/gallery.rst

@@ -81,3 +81,6 @@ Gallery
 
 .. image:: _static/img/viz_thumbnails/separator.png
    :scale: 25 %
+
+.. image:: _static/img/viz_thumbnails/histogram.png
+   :scale: 25 %