Adding a TrendLine is very similar to adding a second data series. C1Chart will automatically calculate and plot the trendline based on the values. By default, a trendline will have a Polynomial FitType with and Order of 2. The following code demonstrates adding a TrendLine:

'add trend line
Dim tl As New TrendLine()
tl.Label = "Trendline"
tl.ConnectionStroke = New SolidColorBrush(Colors.Red)
tl.XValuesSource = myXValues
tl.ValuesSource = myValues
chart.Data.Children.Add(tl)

//add trend line
TrendLine tl = new TrendLine();
tl.Label = "Trendline";
tl.ConnectionStroke = new  SolidColorBrush(Colors.Red);
tl.XValuesSource = myXValues;
tl.ValuesSource = myValues;
chart.Data.Children.Add(tl);

You can see in the following image that the default trendline doesn't fit the data closely:

You can get a better fit by changing the Order property. You can specify both the FitType and the Order properties to get the fit you're looking for:

TrendLine tl = new TrendLine();
tl.Label = "Trend";
tl.FitType = FitType.Polynom;
tl.Order = 6;

The data used in the pictured chart graphs gas prices with values taken every seven days. With this kind of data, a MovingAverage trendline is more appropriate. You can see how it fits the data in the following image:

When setting up a Moving Average trendline, you must instantiate a new MovingAverage object and set the Period property. This property specifies the number of data points to use for the trendline. Since the gas price data is averaged every 7 days, a Period of 48 averages each year of data:

MovingAverage ma = new MovingAverage();
ma.Label = "Moving Average";
ma.Period = 48;
ma.XValuesSource = days;
ma.ValuesSource = price;
ma.ConnectionStroke = new SolidColorBrush(Colors.Red);
chart.Data.Children.Add(ma);