Cut a chart at specific point in C#

Question

So far, my C# program has been taking real-time input from an COM gate, and draw it on a chart, like so:

// DATA is the input from the COM gate, TIMESTAMP is the time the data is taken  
DataChart.Series["Data"].Points.AddXY(TIMESTAMP, DATA);
// Continue processing

I did not save the data into a separate array or class because they are rather large, and have caused our website namesake before.
Now the program is needed to "cut" the chart between two time points (called StartTime and EndTime) into another chart (called CutChart). Since I still do not want to keep them long term, I tried this:

foreach (Series series in DataChart.Series)
    foreach(DataPoint point in series.Points)
        if (point.XValue <= EndTime && point.XValue >= StartTime) 
            CutChart.Series[DataChart.Series.IndexOf(series)].Points.Add(point);

Yet it does not work (As in, the CutChart, which start with no point, and therefore appear empty, once that code line run, still appear empty, no error or exception recorded).
Strangely enough, when I add points in DataChart wrongly (TIMESTAMP in Y axis instead of X), the line of code above work perfectly.
The way I understand it, to draw a graph, the C# Chart class must be saving the XY coordinate of each point... somewhere. To produce a new graph that is a portion of the old graph between two points in time, just add the points with satisfactory X-value to the new graph. Except I do not know where that "somewhere" is.
Note: I need to draw the "cut" part of DataChart on a new chart, so just zoom in is not quite enough.

Answer 1

I've tested your code successfully so the problem is likely somewhere else. I think you would make your life easier by using the Datasource item instead of having to manipulate points and their coordinates yourself.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        chart1.Series.Add("test");
        chart1.Series[0].ChartType = System.Windows.Forms.DataVisualization.Charting.SeriesChartType.Line;
        chart1.Series[0].Points.AddXY(1, 1);
        chart1.Series[0].Points.AddXY(2, 2);
        chart1.Series[0].Points.AddXY(3, 3);


        chart2.Series.Add("test2");
        chart2.Series[0].ChartType = System.Windows.Forms.DataVisualization.Charting.SeriesChartType.Line;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        foreach (Series series in chart1.Series)
            foreach (DataPoint point in series.Points)
                if (point.XValue >= 2)
                    chart2.Series[chart1.Series.IndexOf(series)].Points.Add(point);
    }
}

Edit: What I mean by using datasource is make use of the binding capability of the Chart to decouple your data from its representation, in short, to not have to copy all data points from one chart to another every time.

for example:

        //this is just dummy data, see https://msdn.microsoft.com/en-us/library/system.windows.forms.datavisualization.charting.chart.datasource(v=vs.110).aspx
        List<Tuple<int, int>> data = new List<Tuple<int, int>>() { new Tuple<int, int>(1,1), new Tuple<int, int>(2,5) }; 

        //this code bind your data source to your chart
        chart1.DataSource = data;
        chart1.Series.Add("test");
        chart1.Series[0].ChartType = System.Windows.Forms.DataVisualization.Charting.SeriesChartType.Line;
        chart1.Series[0].XValueMember = "item1";
        chart1.Series[0].YValueMembers = "item2";
        chart1.DataBind();

        // now to create your "cut" chart you can simply query the data you want from the data source and use the same code as above to create another chart.
        var subdata = data.Where(x => x.Item1 > 1);

Answer 2

You wrote

Strangely enough, when I add points in DataChart wrongly (TIMESTAMP in Y axis instead of X), the line of code above work perfectly.

This may be a hint that the problem is with the data types.

Since the x-values are entered from DateTime variables one might conclude that one can compare them to DateTime variables.

But internally all values, x- and y-values are stored as doubles.

So you need to convert any date you want to use in a comparison to double:

var datetime1 = DateTime.Now;
var dDouble = datetime1.ToOADate();

There is also a conversion from the double values:

var datetime2 = DateTime.FromOADate(dDouble );

or

var datetime3 = DateTime.FromOADate(aDataPoint.XValue);

So your condition may be written as

if (point.XValue <= EndTime.ToOADate() && point.XValue >= StartTime.ToOADate()) 

But it is cleaner to do the conversion only once, of course..:

double dStarttime = StartTime.ToOADate();
double dEndTime = EndTime.ToOADate();
..
..
if (point.XValue <= dEndTime  && point.XValue >= dStarttime ) 

---

Other reasons why a Chart may appear empty are:

• No ChartArea. If you create the chart in code, as oposed to dropping it from the toolbox, it will not have a ChartArea.
• No Points. No matter how nicely you style the axes, there still must be at least one dummy point before anything shows up.