Looking at
https://www.railroadflat.org/weather/check-fetch-times.php?show=info shows:
Settings-weather.php $SITE['graphImageDir']='./davcon' exists; but does not have contents (curr24hourgraph.gif tested). Set WD software to upload contents for proper template operation.
Looking at Settings-weather.php shows
$SITE['graphImageDir'] = './davcon'; // directory location for graph images with trailing /
and that needs a trailing / to be
$SITE['graphImageDir'] = './davcon/'; // directory location for graph images with trailing /
(and that assumes the WD graphs are uploaded to the ./davcon/ directory)
However, I see that
https://www.railroadflat.org/weather/curr24hourgraph.gif does exist, so change Settings-weather.php to
$SITE['graphImageDir'] = './'; // directory location for graph images with trailing /
and the history images should work.