Disable clearing of D3.js brush

Question

I would like to use a D3.js brush to allow users to select a range of values on an axis. By default, clicking outside the brush clears it, so that no range is selected.

However, I would like to adjust this behaviour so that clicking outside the brush doesn't alter the brush extent. In effect, there should be no way to clear the brush, some range should always be selected.

I believe I have to hook into the brush event somehow to disable the clearing, but I don't really know how to go about that.

Here's an example of the kind of interface I'm talking about (Fiddle). When you click to the left or right of the black bar, the brush is cleared and the bar disappears.

How can I disable this behaviour?

Answer 1

d3 brush by design calls 'brushmove()' once a user presses a mouse on the brush element (i.e. on 'mousedown.brush' event). If effectively leads to resetting the previous brush extent.

A possible workaround is to replace the original mousedown.brush handler with the custom one. The custom handler will only call the original handlers once the mouse was moved after initial mousedown.

var brushNode = chart.append("g")
    .call(brush);

brushNode
    .selectAll("rect")
    .attr("y", -10)
    .attr("height", 10);

// store the reference to the original handler
var oldMousedown = brushNode.on('mousedown.brush');

// and replace it with our custom handler
brushNode.on('mousedown.brush', function () {
    brushNode.on('mouseup.brush', function () {
        clearHandlers();
    });

    brushNode.on('mousemove.brush', function () {
        clearHandlers();
        oldMousedown.call(this);
        brushNode.on('mousemove.brush').call(this);
    });

    function clearHandlers() {
        brushNode.on('mousemove.brush', null);
        brushNode.on('mouseup.brush', null);
    }
})

See the demo.