Cross Section
  
  A cross section plot widget is created when the user selects Cross
  section for the mouse mode and depresses a mouse button at the
  beginning location of the desired line, drags the mouse to the
  ending location and releases the mouse button.

  Above the plot:

    On the top left side of the widget the line length, line angle, 
    x starting, center, and end positions as well as the y starting, 
    center, and ending positions are reported.  The user may adjust 
    the profile with the Adjust Cross Section Widget, see below.

    On the top right side are the following buttons and labels:

    Adjust:  activates the Adjust Cross Section Widget which contains 
             three boxes (Start/Stop, Start/Angle, Center/Angle) with 
  	     similiar (or redundant) information.  It is important to
	     remember the significance of each box of information.  If
	     the xstart value is changed in the Start/Stop box, the
	     center, line length, and angle are updated.  If the xstart
             value is changed in the Start/Angle box, the xcenter and xend
	     are updated keeping the angle and length fixed.  In each box
	     the user can Apply the changes or Cancel them.  The user may
	     also use the computed centroid position to define the center 
	     of the line with the Centroid X,Y button in the Center/Angle
	     box.

    Save:    saves the current cross section profile to a text file in X, Y 
             pairs.  Pops up a widget allowing user to enter the output file 
             name.  The starting and ending points and line width are saved 
	     at the beginning of the file.
    
    Print:   saves the current cross section profile plot to an output file.  
  	     The output format may be postscript, encapsulated postscript, 
	     PICT, TIFF, or JPEG.  A widget pops up for the user to enter
	     the filename and select the desired format.

    Help:    displays this file.

    Done:    closes the Cross Section Widget.

    Replot:  redisplays the current line without overlays of the gaussian
             profile, residuals, or FWHM.


    Label:   The Gaussian Flux Volume Fit is printed just above the plot. 
             It is computed as 1.1221 * peak value * (fitted FWHM ^ 2).


  On the plot:

    On the lower right side of the plot the xcursor and ycursor values
    are displayed as the cursor is moved in the plot display.


  Below the plot:

    Overplot: when overplot is selected subsequent profiles may be
	      overplotted on the first.  There are 6 distinct line
	      styles to distinguish each plot.

    Auto:     when selected the scaling for the y-axis will be determined
	      automatically.  The minimum and maximum scale values are 
	      displayed in the YDisplay: Min and Max.The user has no control 
	      over the scaling of the x-axis.

    Log:      when selected the y-axis will be scaled logarithmically.

    Plot options which are not mutually exclusive:
	      Gaussian+Base - overplot sum of model gaussian and baseline.
			      If full gaussian is plotted the color is red, 
			      left fit is yellow, and right fit is green.
	      Gaussian      - overplot model gaussian.  Colors are as above.
	      Base          - overplot baseline with dash/dot line in white.
	      Residual      - overplot residual in white.
	      FWHM          - overplot full width half max and 1/e in white.

    YDisplay Min and Max:  The user may specify the desired minimum and
	      maximum for the y-axis.  If auto is selected the values 
	      determined by the procedure are displayed in these boxes.

    NegPk:    If the desired peak to fit is a minimum the user must specify
	      Negative Peak.

    XFit Min and Max: The user may set the pixels over which the gaussian
	      is fit.  The range of pixels may also be selected by depressing
	      the mouse button at the beginning of the fitted area, dragging
	      the mouse to the end of the fitted area, and releasing the mouse
	      button.  These values are then displayed in the XFit Minimum
	      and Maximum fields.  The Reset button resets the minimum and
	      maximum to 0. and the maximum number of original pixels in
	      the plot - 1.

    Line Width:  The line width is initially defined as one zoomed pixel,
	      i.e., 1./zoom factor.  The user may compute the Mean, Sum,
	      or Median of the line according to the input value to Line 
	      Width and the combination method selected.  The input line
	      width is always in original pixels.

    Base:     While fitting a gaussian function the user may simultaneously
	      fit a constant or linear baseline.  The selections are:
	      None, Constant, and Linear.

    Peak:     The peak value is set to the maximum value in the line
	      between the minimum and maximum x-fit values.  

    Base:     The baseline value is initially set to the intercept of the
	      plot.

    ReCalc:   Recalculates the peak and baseline values over the minimum
	      and maximum x-fit values.

    Calc FWHM:  Calculates the FWHM from the peak and baseline values.

    FWHM:     The FWHM calculated above is recorded in this field.  The
	      user may change this value.

    Fit:      The gaussian may be fit over the entire data between xmin
	      and xmax (displayed in red), over the left side of the data
	      from xmin to peak (displayed in yellow) or over the right side
	      of the data from peak to xmax (displayed in green).

    At the bottom of the widget the measured FWHM, Peak Position, Peak Value,
    and Baseline Value are reported on the left side.  On the right side the
    fitted FWHM, Peak Position, Peak Value, and Baseline Value are reported.
    If a linear baseline is selected the Baseline Value is the function value
    at the peak position.