Correct name for waterfall chart with data point pairs showing net value per pair? The chart illustrated below is an attempt to show a concept I'm looking to visualize. On each day I have two numbers I want to show - an increase (tasks assigned), and a decrease (tasks completed). I want to visualize this each day with the net value being where the waterfall leaves off/picks up each day. Some days there might not be a negative value, and some days there might not be a positive value.
Is there a "correct" name for this kind of chart so I can search for charting libraries that might exist for the web? (eg. within the d3.js ecosystem) Of course, if anyone knows of an online charting library for this, I welcome that in the answer too!

 A: This sort of graph doesn't seem much different to me than a normal waterfall graph. The only difference is you've added a little bit of extra spacing between every other bar and there are two points corresponding to each tick on the x axis. 
But look here, I've recreated your graph, including your notes here using HighCharts, I couldn't make it work how I'd like d3 unfortunately, but highcharts is a great library.
By assigning each date two values and using the tickInterval: 2 parameter to hide every other tick label, it looks pretty good if you ask me.

This is the code
$(function () {
    $('#container').highcharts({
        chart: {
            type: 'waterfall'
        },
        title: {
            text: 'Task Given/Completed'
        },
        xAxis: {
            title: {
                text: 'Date'
            },
            type: 'category',
            tickInterval: 2,
        },
        yAxis: {
            title: {
                text: 'Number of tasks'
            }
        },
        legend: {
            enabled: false

        },
        series: [{
            upColor: Highcharts.getOptions().colors[2],
            color: Highcharts.getOptions().colors[8],
            data: [{
                name: '2015-01-01',
                y: 50
            }, {
                name: '2015-01-01',
                y: -40
            }, {
                name: '2015-01-02',
                y: 60
            }, {
                name: '2015-01-02',
                y: -34
            }, {
                name: '2015-01-03',
                y: 90
            }, {
                name: '2015-01-03',
                y: -16
            }, {
                name: '2015-01-04',
                y: 80
            }, {
                name: '2015-01-04',
                y: -100
            }, {
                name: '2015-01-05',
                y: 140
            }, {
                name: '2015-01-05',
                y: -0
            }, {
                name: '2015-01-06',
                y: 60
            }, {
                name: '2015-01-06',
                y: -34
            }, {
                name: '2015-01-07',
                y: 0
            }, {
                name: '2015-01-07',
                y: -34
            }, {
                name: '2015-01-08',
                y: 80
            }, {
                name: '2015-01-08',
                y: -34
            }, {
                name: 'Remaining',
                isSum: true,
                color: Highcharts.getOptions().colors[0]

            }],
            dataLabels: {
                style: {
                    color: '#FFFFFF',
                    fontWeight: 'bold',
                    textShadow: '0px 0px 3px black'
                }
            },
            pointPadding: 0
        }]
    });
});

And here it is on jsfiddle: http://jsfiddle.net/qpsbhjvg/
Hope that's what you're looking for.
Edit: 
If you wanted those extra spaces in between bars you could do this, but I wouldn't recommend it because it seems sort of hokey. I tried to use the grouping feature (see here), but it doesn't work like you'd want, so instead I just added null datapoints between days. 

Here this one is on jsfiddle: http://jsfiddle.net/tapb41an/4/
