This is a utility function that allows the user to add a line threshold with text annotation using an intercept parameter. The shift parameter allows the user to relatively adjust the text with respect to the defined thresholding.
Arguments
- intercept
- x or y intercept (indicated by axis) through which the vertical or horizontal threshold is made. 
- axis
- Axis along which a threshold is to be added. Default is "x" indicating a vertical threshold 
- label
- Label to add to the graph. Defaults to an empty string 
- shift
- Variable to shift the label by "shift" units above thresholding line along "y", or by "shift" units to the right of thresholding line along "x"