I’m using GIFgraph to create a line graph indicating the level of a lake from June 30, 1867 to the present. From 1931 I have monthly values. Prior to that the level was measured intermittently. From my research on GIFgraph (this is my first project with it), I learned that the x values (the dates in this case) aren’t treated truly as numeric. It assumes even spacing between the x-values. (If that is not the case, please tell me.) So, for the months prior to 1931 with no data, I entered the dates with “undef” for the level.

When I generate the graph, everything from 1931 on is great. Prior to that, the months with data are indicated by a vertical line starting at the bottom of the graph image, passing through the appropriate date on the x-axis and continuing on to the correct level as indicated on the y-axis. So from 1867 to 1931, I have vertical lines wherever there is data. From 1931, I have a line graph connecting the monthly lake values.

I know it is not connecting the values prior to 1931 because of all the undef. I there a way I can get those points to appear as points and everything after that as a line graph? Or even get the vertical line to just go from the x-axis to the appropriate y-value, rather than the bottom of the whole image to the y-value? Or does anybody know the appropriate way to handle such data with GIFgraph? 

#!/usr/bin/perl
     2
     3  use strict;

    26  # line chart class
    27  use GIFgraph::lines;
    28  use GD;
    29
    30
    31  # create a new object
    32  my $graph = new GIFgraph::lines(600,400);
    33
    34  $graph->set_title_font(gdMediumBoldFont);
    35  $graph->set_x_label_font(gdMediumBoldFont);
    36  $graph->set_y_label_font(gdMediumBoldFont);
    37  $graph->set_x_axis_font(gdMediumBoldFont);
    38  $graph->set_y_axis_font(gdMediumBoldFont);
    39  $graph->set_legend_font(gdMediumBoldFont);
    40  $graph->set(logo=>'80x22_green.gif',logo_resize=>.8,logo_posit
+ion=> 'LR');
    41  $graph->set(textclr=>'black',labelclr=>'black',axislabelclr=>'
+black',accentclr=>'black',bgclor=>'white
',fgclr=>'black',transparent=>0,x_label_skip=>300);
    42  $graph->set(t_margin=>10,b_margin=>10,l_margin=>10,r_margin=>1
+0);
    
   55  my @data = (
    56          [ @dates ],
    57          [ @levels ],
    58          [ @overflow ],
    59          [ @low1992 ],
    60  );
    61
    62  my @legend = ( 'Lake Elevation', 'Overflow to Stump Lake', 'La
+ke Level in 1992' );
    63  $graph->set_legend(@legend);
    64
    65  $graph->set (
    66          title => "05056500 Devils Lake near Devils Lake, North
+ Dakota",
    67          x_label => "1867 to Present",
    68          y_label => "Elevation (feet)",
    69          y_max_value => 1455.00,
    70          y_min_value => 1400.00,
    71          y_tick_number => 11,
    72  );
    73
    74  binmode STDOUT;
    75
    76  my $image_name = "dvl_POR.gif";
    77
    78  # Output the graph
    79  $graph->plot_to_gif($image_name, \@data );
    80
    81  print "$image_name was created."

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In reply to GIFGraph and undefined values by kryberg

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