@arseneksa97,
You can use subtitles to show a watermark in the plot area of the chart as shown in the code snippet below:
subtitles: [{
text: "Built using CanvasJS",
fontSize: 40,
verticalAlign: "center",
dockInsidePlotArea: true,
fontColor: "rgba(0,0,0,0.1)"
}],
Please check this JSFiddle for a working example.

—
Thangaraj Raman
Team CanvasJS