;+
; Creates a dichotomous sparkline as a PNG image file.
; 
; @param filename {in}{required}{type=string} filename of PNG file to write
; @param data {in}{required}{type=lonarr} values can be -1, 0, or +1
; @keyword ysize {in}{optional}{type=integer}{default=12} ysize in pixels of 
;          the output image
; @keyword color {in}{optional}{type=bytarr(3) or index}
;          {default=[0, 0, 0] or 0} color of the plot
; @keyword background {in}{optional}{type=bytarr(3) or index}
;          {default=[255, 255, 255] or 255} background color for the plot
;-
pro mg_sparkdichotomous, filename, data, ysize=ysize, $
                         color=color, background=background
    compile_opt strictarr

    n_data = n_elements(data)
    multiplier = 1L

    my_xsize = 2 * n_data - 1L
    my_ysize = n_elements(ysize) eq 0 ? 12 : ysize 

    my_color = n_elements(color) eq 0 ? bytarr(3) : color 
    my_background = n_elements(background) eq 0 ? bytarr(3) + 255B : background

    band = bytarr(my_xsize, my_ysize)

    for i = 0L, n_data - 1L do begin
        case 1B of
            data[i] lt 0 : begin
                bar_min = 0
                bar_max = my_ysize / 2 - 1L
            end
            data[i] gt 0 : begin
                bar_min = my_ysize / 2
                bar_max = my_ysize - 1L
            end
            else : begin
                bar_min = my_ysize / 2 
                bar_max = my_ysize / 2
            end
        endcase

        band[2*i, bar_min:bar_max] = 1B
    endfor

    red = [my_background[0], my_color[0]]
    green = [my_background[1], my_color[1]]
    blue = [my_background[2], my_color[2]]

    alpha = [0B, 255B]
    hasAlpha = n_elements(background) eq 0

    image = bytarr(3 + hasAlpha, my_xsize, my_ysize)
    image[0, *, *] = red[band]
    image[1, *, *] = green[band]
    image[2, *, *] = blue[band]
    if (hasAlpha) then image[3, *, *] = alpha[band]

    write_png, filename, image
end