Click on the Options > Trace Plots command or press the F3 key to toggle on or off plot trace mode.


If you move the cross to a location that appears to be "on" a plot, it may be near but not quite on the plot.  To more accurately display the coordinate values of a specific point on the plot, turn on trace mode.  If trace mode is on, the cross is confined to moving along (or tracing) the plot from point to point while displaying the corresponding coordinate values in the status bar.


When you switch to trace mode the cross will move up or down to the plot line at the current x-coordinate.  In trace mode the cross displays as a small square unfilled box, whose center lies on the plot line.  The expression number of the plot being traced is displayed in the 2D-plot window title bar.  This makes trace mode useful for distinguishing plot lines when plotting multiple expressions (see Multiple PlotsMultiple_Plots).


Once in trace mode, use the direction keys or the mouse to move the box along the plot as follows:

•        Left arrow or click the right mouse button and choose Move left one unit to move the box left one pixel and up or down as necessary to stay on the plot.

•        Right arrow or click the right mouse button and choose Move right one unit to move the box right one pixel and up or down as necessary to stay on the plot.

•        Ctrl+Left arrow or click the right mouse button and choose Move left multiple units to move the box left half a tick mark and up or down as necessary to stay on the plot.

•        Ctrl+Right arrow or click the right mouse button and choose Move right multiple units to move the box right half a tick mark and up or down as necessary to stay on the plot.


The Up arrow and Down arrow keys as well as the mouse can be used to trace another plot line in a 2D-plot window with multiple expressions plotted (see Multiple PlotsMultiple_Plots).


Alternatively, the trace box may be moved to a specific plot value using the mouse.  Simply click the left mouse button at the desired x-coordinate to move the box to the corresponding plot value at that coordinate.


In trace mode the Set > Cross Position command9IA1L7 moves the box to the point specified by the cross coordinates, regardless of whether this point is on the plot line.  To move the box back to the plot line and continue tracing use the direction keys or the mouse as explained previously.


When trace mode is turned off the cross will return to its original shape and the cross direction keys to their original use.


All 2D plots may be traced except for Implicit PlotsImplicit_Plots).  See Polar PlotsPolar_Plots for specifics about tracing in polar coordinates.  If you try to turn on trace mode, hear an error beep and the cross does not move to the plot line, either there is no explicit plot to trace or the expression is not real and finite at the cross’s current x-coordinate.  Move the cross left or right to where the expression is real and finite and try again to switch to trace mode.


Tracing a plot is especially helpful in visually locating intercepts and minimum and maximum expression values.  If, while tracing along a plot, you encounter a point at which the expression does not have a finite real value, the message 

Tracing expression #n, not real and finite!

is displayed in the 2D-plot window title bar and a warning error beep occurs.  This makes tracing useful for locating removable singularities that may not be apparent on the plot.  If you continue moving the cross, the message will remain until the expression has a finite, real value.


When tracing a plot, it may be advantageous to turn on follow cross mode using the Options > Follow Cross command4V32.T.  If follow cross mode is off and you trace to a point on the plot that extends past the window borders the cross will disappear from sight.  However, the coordinate values of the cross will continue to be updated in the status bar.  If follow cross mode is on while you are tracing a plot, the window will be scrolled to follow the cross as it moves outside the 2D-plot window.


Other 2D-plot Window Options commands31KXD7M 

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