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    $self->{'_imy'} = $_[1];
  }
  else {
    $self->{'_imx'} = 400;
    $self->{'_imy'} = 300;
  }

  # set graph offset; graph will be centered this many pixels within  image
  $self->{'_horGraphOffset'} = 50;
  $self->{'_vertGraphOffset'} = 50;

  # create an empty hash for the datsets
  # data sets and their styles are hashes whose keys are 1 ... _numDataSets
  # and values are refs to flat data arrays or style strings, respectively
  $self->{'_data'} = {};
  $self->{'_dataStyle'} = {};
  $self->{'_numDataSets'} = 0;

  # calculated by _getMinMax and used in translating _data2pxl()
  $self->{'_xmin'} = 0;    $self->{'_xmax'} = 0; # among all datasets
  $self->{'_ymin'} = 0;    $self->{'_ymax'} = 0;
  $self->{'_xslope'} = 0;  $self->{'_yslope'} = 0; # for _data2pxl()
  $self->{'_ax'} = 0;      $self->{'_ay'} = 0;
  $self->{'_omx'} = 0;     $self->{'_omy'} = 0; # for axis ticks
  $self->{'_validMinMax'} = 0; # last calculated min and max still valid

  # initialize text
  ($self->{'_horAxisLabel'}, $self->{'_vertAxisLabel'}) = (q{},q{});
  $self->{'_title'} = q{};
  $self->{'_errorMessage'} = q{};

  # initialize custom tick labels
  ($self->{'_xTickLabels'}, $self->{'_yTickLabels'}) = (0,0);

  # undocumented: in script, use as $plotObject->{'_debugging'} = 1;
  $self->{'_debugging'} = 0;
}

sub _init_gd {
  my $self = shift;

  use GD;

  #  create an image object
  $self->{'_im'} = new GD::Image($self->{'_imx'}, $self->{'_imy'});

  # find format(s) supported by GD
  unless (@_image_types) {
    for ( qw(png gif jpeg) ) {
      push @_image_types, $_ if $self->{'_im'}->can($_);
    }
  }

  #  allocate some colors
  $self->{'_white'} = $self->{'_im'}->colorAllocate(255,255,255);
  $self->{'_black'} = $self->{'_im'}->colorAllocate(0,0,0);
  $self->{'_red'} = $self->{'_im'}->colorAllocate(255,0,0);
  $self->{'_blue'} = $self->{'_im'}->colorAllocate(0,0,255);
  $self->{'_green'} = $self->{'_im'}->colorAllocate(0,255,0);

  # make the background transparent and interlaced
  $self->{'_im'}->transparent($self->{'_white'});
  $self->{'_im'}->interlaced('true');

  # Put a black frame around the picture
  $self->{'_im'}->rectangle( 0, 0,
                             $self->{'_imx'}-1, $self->{'_imy'}-1,
                             $self->{'_black'});
}

sub _init_cv {
  my $self = shift;

  use Tk;
  my($widget) = @_;

  #  create an canvas object
  $self->{'_cv'} = $widget->Canvas(
      -width        => $self->{'_imx'},
      -height       => $self->{'_imy'},
  );

  # make the background white
  $self->{'_cv'}->configure(
      -background   => 'white',
  );

  # some fonts
  if ($^O eq 'MSWin32') {
    $self->{'_MediumBoldFont'} = "{MS Sans serif} 8 bold";
    $self->{'_SmallFont'} = "Tahoma 8";
    $self->{'_TinyFont'} = "{Small Fonts} 6";
  }
  else {
    $self->{'_MediumBoldFont'} = '7x13bold';
    $self->{'_SmallFont'} = '6x12';
    $self->{'_TinyFont'} =  '5x8';
  }
}

# draws all the datasets in $self->{'_data'}
# usage: $self->_createData()
sub _createData {
  my $self = shift;
  my ($i, $num, $px, $py, $prevpx, $prevpy, $dataSetLabel, $color);

  foreach $dataSetLabel (keys %{$self->{'_data'}}) {

    # get color
    if ( $self->{'_dataStyle'}->{$dataSetLabel} =~ /((red)|(blue)|(green))/i ) {
      $color = $1;
      $color =~ tr/A-Z/a-z/;
    }
    else {
      $color = 'black';
    }

    # draw the first point
    ($px, $py) = $self->_data2pxl (
                                   $self->{'_data'}->{$dataSetLabel} [0],
                                   $self->{'_data'}->{$dataSetLabel} [1]
                                  );