Can I plot a colorbar for a bokeh heatmap?

Question

Does bokeh have a simple way to plot the colorbar for a heatmap?

In this example it would be a strip illustrating how colors correspond to values.

Answer 1

This is high on my wish list as well. It would also need to automatically adjust the range if the plotted data changed (e.g. moving through one dimension of a 3D data set). The code below does something which people might find useful. The trick is to add an extra axis to the colourbar which you can control through a data source when the data changes.

import numpy

from bokeh.plotting import Figure

from bokeh.models import ColumnDataSource, Plot, LinearAxis
from bokeh.models.mappers import LinearColorMapper
from bokeh.models.ranges import Range1d
from bokeh.models.widgets import Slider
from bokeh.models.widgets.layouts import VBox

from bokeh.core.properties import Instance

from bokeh.palettes import RdYlBu11

from bokeh.io import curdoc

class Colourbar(VBox):

    plot = Instance(Plot)
    cbar = Instance(Plot)

    power = Instance(Slider)

    datasrc = Instance(ColumnDataSource)
    cbarrange = Instance(ColumnDataSource)

    cmap = Instance(LinearColorMapper)

    def __init__(self):

        self.__view_model__ = "VBox"
        self.__subtype__ = "MyApp"

        super(Colourbar,self).__init__()

        numslices = 6
        x = numpy.linspace(1,2,11)
        y = numpy.linspace(2,4,21)
        Z = numpy.ndarray([numslices,y.size,x.size])
        for i in range(numslices):
            for j in range(y.size):
                for k in range(x.size):
                    Z[i,j,k] = (y[j]*x[k])**(i+1) + y[j]*x[k]

        self.power = Slider(title = 'Power',name = 'Power',start = 1,end = numslices,step = 1,
                            value = round(numslices/2))
        self.power.on_change('value',self.inputchange)

        z = Z[self.power.value]
        self.datasrc = ColumnDataSource(data={'x':x,'y':y,'z':[z],'Z':Z})

        self.cmap = LinearColorMapper(palette = RdYlBu11)

        r = Range1d(start = z.min(),end = z.max())        
        self.cbarrange = ColumnDataSource(data = {'range':[r]})

        self.plot = Figure(title="Colourmap plot",x_axis_label = 'x',y_axis_label = 'y',
                           x_range = [x[0],x[-1]],y_range=[y[0],y[-1]],
                           plot_height = 500,plot_width = 500)

        dx = x[1] - x[0]
        dy = y[1] - y[0]

        self.plot.image('z',source = self.datasrc,x = x[0]-dx/2, y = y[0]-dy/2,
                        dw = [x[-1]-x[0]+dx],dh = [y[-1]-y[0]+dy],
                        color_mapper = self.cmap)

        self.generate_colorbar()

        self.children.append(self.power)
        self.children.append(self.plot)
        self.children.append(self.cbar)

    def generate_colorbar(self,cbarlength = 500,cbarwidth = 50):

        pal = RdYlBu11

        minVal = self.datasrc.data['z'][0].min()
        maxVal = self.datasrc.data['z'][0].max()
        vals = numpy.linspace(minVal,maxVal,len(pal))

        self.cbar = Figure(tools = "",x_range = [minVal,maxVal],y_range = [0,1],
                           plot_width = cbarlength,plot_height = cbarwidth)

        self.cbar.toolbar_location = None 
        self.cbar.min_border_left = 10
        self.cbar.min_border_right = 10
        self.cbar.min_border_top = 0
        self.cbar.min_border_bottom = 0
        self.cbar.xaxis.visible = None
        self.cbar.yaxis.visible = None
        self.cbar.extra_x_ranges = {'xrange':self.cbarrange.data['range'][0]}
        self.cbar.add_layout(LinearAxis(x_range_name = 'xrange'),'below')

        for r in self.cbar.renderers:
            if type(r).__name__ == 'Grid':
                r.grid_line_color = None

        self.cbar.rect(x = vals,y = 0.5,color = pal,width = vals[1]-vals[0],height = 1)

    def updatez(self):

        data = self.datasrc.data
        newdata = data
        z = data['z']
        z[0] = data['Z'][self.power.value - 1]
        newdata['z'] = z
        self.datasrc.trigger('data',data,newdata)

    def updatecbar(self):

        minVal = self.datasrc.data['z'][0].min()
        maxVal = self.datasrc.data['z'][0].max()
        self.cbarrange.data['range'][0].start = minVal
        self.cbarrange.data['range'][0].end = maxVal

    def inputchange(self,attrname,old,new):

        self.updatez()
        self.updatecbar()

curdoc().add_root(Colourbar())