plotAddArrow
==============================================

Purpose
----------------
Adds an arrow to an existing graph.

Format
----------------
.. function:: plotAddArrow([myAnnotation, ]x_start, y_start, x_end, y_end, head_size)

    :param myAnnotation: Optional argument. An instance of a :class:`plotAnnotation` structure.
    :type myAnnotation: struct

    :param x_start: the X coordinate for the start of each respective arrow.
    :type x_start: scalar or Nx1 vector

    :param y_start: the Y coordinate for the start of each respective arrow.
    :type y_start: scalar or Nx1 vector

    :param x_end: the X coordinate for the end of each respective arrow.
    :type x_end: scalar or Nx1 vector

    :param y_end: the Y coordinate for the end of each respective arrow.
    :type y_end: scalar or Nx1 vector

    :param head_size: the size of the arrowhead(s) in pixels. The first element of *head_size* is the size for
        head at the end of the arrow. The second element is the size of the head at the start of the arrow.
    :type head_size: 2x1 vector

Examples
----------------

Basic usage
+++++++++++

::

    x_start = 0.2;
    y_start = 0.25;
    x_end = 0.4;
    y_end = 0.5;

    // Set arrowhead at the end to 15 px
    // No arrowhead at the beginning of the arrow
    head_size = { 15, 0 };

    // Add an arrow to graph
    plotAddArrow(x_start, y_start, x_end, y_end, head_size);

Add an arrow between points
+++++++++++++++++++++++++++

::

    // Draw random scatter plot
    x = rndu(10, 1);
    y = rndu(10, 1);
    plotScatter(x, y);

    // Add arrow from the first point to the ninth point
    plotAddArrow(x[1], y[1], x[9], y[9], 12);

Remarks
-------

Please note that :func:`plotAddArrow` will add arrows to existing graphs, it
will not create a new graph if one does not exist. :func:`plotAddArrow` is not
yet supported for surface plots.


.. seealso:: Functions :func:`plotAddTextbox`, :func:`annotationGetDefaults`, :func:`annotationSetLineColor`