DateTime Axis - ScottPlot FAQ

C# can represent time with the DateTime type, but all of ScottPlot’s methods prefer double arrays. This page reviews how to plot data with a time axis using ScottPlot.

💡 TLDR: Convert DateTime to double using DateTime.ToOATime() before plotting, then call XAxis.DateTimeFormat(true) to display axis tick labels in the proper format.

Scatter Plot

If time points are not evenly spaced, the Scatter Plot should be used (otherwise a Signal plot should be considered).
Use LINQ and DateTime.ToOATime() to convert DateTime[] to double[] before plotting
Call XAxis.DateTimeFormat(true) to indicate axis tick labels should be formatted as DateTime values (instead of the default numeric format)

// generate sample data (arrays of individual DateTimes and values)
int pointCount = 100;
Random rand = new Random(0);
double[] values = ScottPlot.DataGen.RandomWalk(rand, pointCount);
DateTime[] dates = Enumerable.Range(0, pointCount)
                              .Select(x => new DateTime(2016, 06, 27).AddDays(x))
                              .ToArray();

// use LINQ and DateTime.ToOADate() to convert DateTime[] to double[]
double[] xs = dates.Select(x => x.ToOADate()).ToArray();

// plot the double arrays using a traditional scatter plot
var plt = new ScottPlot.Plot(600, 400);
plt.AddScatter(xs, values);

// indicate the horizontal axis tick labels should display DateTime units
plt.XAxis.DateTimeFormat(true);

// add padding to the right to prevent long dates from flowing off the figure
plt.YAxis2.SetSizeLimit(min: 40);

// save the output
plt.Title("Scatter Plot with DateTime Axis");
plt.SaveFig("../../../datetime-scatter.png");

Source code for this application is available on GitHub

Signal Plot

Signal plots offer a huge performance increase over Scatter Plots
Signal plots calculate X position from sample rate and offset
The value of sample rate is the number of samples per day:
- daily: sampleRate = 1
- hourly: sampleRate = 24
- minute: sampleRate = 24 * 60
- second: sampleRate = 24 * 60 * 60
- millisecond: sampleRate = 24 * 60 * 60 * 1000

// generate sample data (array of values with a single starting DateTime)
int pointCount = 100;
Random rand = new Random(1);
double[] values = ScottPlot.DataGen.RandomWalk(rand, pointCount);
DateTime firstDay = new DateTime(2016, 06, 27);
double samplesPerDay = (24 * 60); // one sample per minute

// plot the double arrays using a traditional scatter plot
var plt = new ScottPlot.Plot(600, 400);
var sig = plt.AddSignal(values, sampleRate: samplesPerDay);
sig.OffsetX = firstDay.ToOADate();

// indicate the horizontal axis tick labels should display DateTime units
plt.XAxis.DateTimeFormat(true);

// add padding to the right to prevent long dates from flowing off the figure
plt.YAxis2.SetSizeLimit(min: 40);

// save the output
plt.Title("Signal Plot with DateTime Axis");
plt.SaveFig("../../../datetime-signal.png");

Notes

👉 The ScottPlot Cookbook has additional information and code examples demonstrating advanced DateTime tick label functionality
Axes have additional options for tick formatting, including the ability to define format strings for the date-to-string conversion, as well as the ability to indicate which culture to use (e.g., different cultures periods, hyphens, or dashes to separate month and year).