@hardworkmedicina,
You can use the toolTipContent property to customize the content of the toolTip and add a prefix to the x-value as shown in the code snippet below,
toolTipContent: " Simulando {x} <br/> <span style='\"'color: {color};'\"'>{name}:</span> {y}",
Kindly take a look at this JSFiddle for an example on customizing the toolTip content using toolTipContent property.

—
Adithya Menon
Team CanvasJS