Plotting: Fix histogram plotting.

Plotting: Fix histogram plotting.

William Henney writes:

     The following table works correctly with org-plot
    #+PLOT: title:"org-plot test" ind:1 type:2d with:hist set:"style fill
    solid" set:"yrange [0:]"
     |  Year | a | b |
     |-------+---+---|
     | x2006 | 3 | 1 |
     |  2007 | 1 | 2 |
     |  2008 | 2 | 0 |

     However, if the "x" is removed from the start of all the years, it no
     longer works:

    #+PLOT: title:"org-plot bug" ind:1 type:2d with:hist set:"style fill
    solid" set:"yrange [0:]"
     | Year | a | b |
     |------+---+---|
     | 2006 | 3 | 1 |
     | 2007 | 1 | 2 |
     | 2008 | 2 | 0 |

     From glancing through org-plot.el, it seems as though the problem is
     that the text-ind parameter is false when all the values in the
     "independent variable" column are legal numbers. However, my lisp
     skills are not up to fixing this.

This patch is by Charles Sebold and Eric Schulte fixes the problem.