Question

I have a simple line chart being fed from a web socket connection and i’m applying a “monotone” filter to smooth out the lines, so to avoid seeing the lines adjust as new data comes in I am clipping the chart to hide the most recent data points as advised in this article…

http://bost.ocks.org/mike/path/

but this makes my axis look incorrect, the right edge has a gap showing the difference between the clip rect and the real output domain as you can see…

I have been able to fix this by adding a different x scale that reduces the size of the domain to the clip rect but that seems hacky to me, and not a particularly clean solution.

Is there a correct way to fix this?

Here is an abbreviated code listing showing the relevant portions…

// Create an x-scale
var x = d3.scale.linear()
    .domain([0, saved_points])
    .range([0, width - margin]);

// Create the axis
var xAxis = d3.svg.axis()
            .scale(x)
            .tickSize(-height)
            .tickValues([10,20,30,40,50,60,70,80,90]);

// Clip path truncates the last two points from the line, because adding new
// control points alters the shape of the line, and it "wiggles"
chart.append("defs")
    .append("clipPath")
    .attr("id", "clip")
    .append("rect")
    .attr("width", width - margin - x(2))
    .attr("height", height);

// Create the stack of lines
y_bands = d3.scale.ordinal().rangeBands([0,height]);
line = d3.svg.line()
    .x(function(d,i){ return x(i); })
    .y(function(d,i){
        var a = -1.0 * (y(d.value) / y_bands.domain().length);
        var b = y_bands(d.name);
        var result = a + height - b;
        return result;
    })
    .interpolate("monotone");

// Put the Axis at the bottom of the graph
d3.select("svg")
    .append("svg:g")
    .attr("class", "xaxis")
    .attr("transform", "translate(0," + (height) + ")")
    .call(xAxis);

// Finally create all the paths
chart.selectAll("path")
    .data(my_line_chart.values)
    .enter()
    .append("g")
    .attr("clip-path", "url(#clip)")
    .append("svg:path")
    .attr("class", "line_chart")
    .attr("stroke", function(d, i) { return color(i); })
    .attr('d', function(d,i){ return line(my_line_chart.values[i]);} );

Answer 1

I haven’t found an ideal solution for this but the technique I mentioned in my question does work.

By creating a second scale that matches the clip path size instead of the actual data range I can draw the axis with that and have them match up.

        // define actual scales
        var x = d3.scale.linear()
            .domain([0, saved_points])
            .range([0, width - margin]);

        // Define a scale that's reduced in size of the output range by the
        //  length of two points of the actual scale.
        var fake_x = d3.scale.linear()
            .domain([0, saved_points - 2])
            .range([0, width - margin - x(2)]);

        // Use this fake scale for the axis instead of the real scale.
        var xAxis = d3.svg.axis()
                    .scale(fake_x)
                    .tickSize(-height)
                    .tickValues(ticks);

        // The clip path uses the same width reduced by two points
        chart.append("defs")
            .append("clipPath")
            .attr("id", "clip")
            .append("rect")
            .attr("width", width - margin - x(2))
            .attr("height", height);

The only disadvantage i’ve seen to this method is that animating the width has issues because that x(2) value in the fake_x scale doesn’t transition.