Will allow you to plot points that wont plot a point on the line
chart1.Series[dataTable.Columns[x].Caption].Points.Add(new DataPoint(dataTable.Rows[i][0], double.NaN) { IsEmpty = true });
if you don't want to plot values less than an hour try an if statement in your for loop that will control the points plotted
for (int i = 0; i < dataTable.Rows.Count - 1; i++)
//assuming dataTable.Rows[i][0] is time, then if the the value is in the
//last hour [DateTime.Now.AddHours(-1)] the if statement will allow the
//point to be plotted, otherwise it wont plot
if (dataTable.Rows[i][0] > DateTime.Now.AddHours(-1)) {
chart1.Series[dataTable.Columns[x].Caption].Points.AddXY(dataTable.Rows[i][0], dataTable.Rows[i][x]);
}
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