Can the colors of bars in a bar chart be varied based on their value? - chart.js

Question

Using chart.js 2, can the colors of the bars in a bar-cart be varied based on their value?
For example, if the scale is 0 - 100, columns with 50% and above could be green, while 0-49% could be red.

Answer 1

As far as I know there is no configuration or callback for each individual point being drawn. The best way I can think of to do this would be to create a function that would modify your chart config/data object. This isn't the most elegant way to deal with the problem, but it would work.
The Fix
Pass your chart config/data object to a function that will add the background color.
Main Point of the example is function AddBackgroundColors(chartConfig)
Example:
function AddBackgroundColors(chartConfig) {
var min = 1; // Min value
var max = 100; // Max value
var datasets;
var dataset;
var value;
var range = (max - min);
var percentage;
var backgroundColor;
// Make sure the data exists
if (chartConfig &&
chartConfig.data &&
chartConfig.data.datasets) {
// Loop through all the datasets
datasets = chartConfig.data.datasets;
for (var i = 0; i < datasets.length; i++) {
// Get the values percentage for the value range
dataset = datasets[i];
value = dataset.data[0];
percentage = value / range * 100;
// Change the background color for this dataset based on its percentage
if (percentage > 100) {
// > 100%
backgroundColor = '#0000ff';
} else if (percentage >= 50) {
// 50% - 100%
backgroundColor = '#00ff00';
} else {
// < 50%
backgroundColor = '#ff0000';
}
dataset.backgroundColor = backgroundColor;
}
}
// Return the chart config object with the new background colors
return chartConfig;
}
var chartConfig = {
type: 'bar',
data: {
labels: ["percentage"],
datasets: [{
label: '100%',
data: [100]
}, {
label: '50%',
data: [50]
}, {
label: '49%',
data: [49]
}, {
label: '5%',
data: [5]
}]
},
options: {
scales: {
yAxes: [{
ticks: {
beginAtZero: true
}
}]
}
}
};
window.onload = function() {
var ctx = document.getElementById("canvas").getContext("2d");
chartConfig = AddBackgroundColors(chartConfig);
var myChart = new Chart(ctx, chartConfig);
};
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.1.2/Chart.bundle.min.js"></script>
<canvas id="canvas" width="400" height="200"></canvas>

Answer 2

In Chart.js 2 it is possible to set multiple colors with an array.
So you can define the backgroundColor as an array of color strings, matching the datasets data.
var myChart = new Chart(ctx, {
datasets: [{
label: 'Votes',
data: [1, 2, 3],
// Make the first bar red, the second one green and the last one blue
backgroundColor: ['#f00', '#0f0', '#00f']
}]
});
You can easily generate an array based on the values in data:
function getColorArray(data, threshold, colorLow, colorHigh) {
var colors = [];
for(var i = 0; i < data.length; i++) {
if(data[i] > threshold) {
colors.push(colorHigh);
} else {
colors.push(colorLow);
}
}
return colors;
}
See this fiddle for a working demo