Incorrect empty datapoints display in a fastline chart

Question

I have a fast-line chart series where on X I have DateTime and on Y double values - series is added to the chart with such method:

public virtual bool AddOrUpdateSeries(int caIndex, Series newSeries, bool visibleInLegend)
{
    var chartArea = GetChartArea(caIndex);

    if (chartArea == null) return false;

    var existingSeries = _chart.Series.FirstOrDefault(s => s.Name == newSeries.Name);

    if (existingSeries != null)
    {
        existingSeries.Points.Clear();
        AddPoints(newSeries.Points, existingSeries);
    }
    else
    {
        newSeries.ChartArea = chartArea.Name;
        newSeries.Legend = chartArea.Name;
        newSeries.IsVisibleInLegend = visibleInLegend;
        newSeries.BorderWidth = 2;
        newSeries.EmptyPointStyle = new DataPointCustomProperties { Color = Color.Red };

        _chart.Series.Add(newSeries);
    }

    return true;
}

As you can see, I am setting the style for empty point to be shown in red.

The first points that are added to the series are as follows:

So as you can see, first two points have the same Y value, but in addition - first one has IsEmpty flag set.

Empty point is added to the series with such piece of code:

 series.Points.Add(new DataPoint
 {
    XValue = _beginOADate, 
    YValues = new[] { firstDbPoint.Y }, 
    IsEmpty = true
 });

where _beginOADate is double OADate value = 42563 = 12/07/2016 00:00 as DateTime.

The second point's DateTime is 15/08/2016 22:20

When chart is displayed with the beginning of the X axis, everything looks ok as on the picture below - empty datapoint starts at 12/07/2016 and lasts until 15/08/2016.

But, when I scroll one position on X, the empty datapoint's red line is not being displayed - instead, whole visible part of empty datapoint's line is displayed as it is non-empty:

Anybody knows how to fix this behaviour so that the whole line starting from Empty datapoint until first non-empty datapoint would always be shown in red?

Of course the dummy solution would be to add one more extra empty datapoint very close to the first non-empty point, but I don't like that solution.

Fastline doesn't support all features; does the problem persist when changing to Line? — TaW
Well, when I changed it to Line everything works AS EXPECTED - Thanks TaW, but the rendering of the chart is much slower. I have charts with 30k points so when zoomed it slows down quite too much. Anyone knows how to make it more smooth? — pitersmx
@TaW Can you post your comment as an answer? Actually it solves the problem. It might be useful to the others. — pitersmx

TaW TaW · Accepted Answer · 2017-07-12T21:02:25

The ChartType.FastLine is much faster the the simple Line chart but to be so fast it makes several simplifications in rendering, which means that not all chart features are supported:

The FastLine chart type is a variation of the Line chart that significantly reduces the drawing time of a series that contains a very large number of data points. Use this chart in situations where very large data sets are used and rendering speed is critical.

Some charting features are omitted from the FastLine chart to improve performance. The features omitted include control of point level visual attributes, markers, data point labels, and shadows.

Unfortunately EmptyPointStyle is such a 'point level visual attribute'.

So you will need to decide which is more important: The raw speed or the direct and plausible treatment of empty DataPoints.

(I have a hunch that you'll go for your 'dummy solution', which imo is a 1st class workaround ;-)

Incorrect empty datapoints display in a fastline chart

1 Answers