An example of observation chart used in the Billings Ovulation Method for natural family planning.
<table style="margin: 1.5em auto; width:60%; background-color:#CCCCCC; border:2px solid #aaaaaa; padding:1px;" cellspacing="10"><tbody><tr><td><i><a href="https://upload.wikimedia.org/wikipedia/commons/3/36/Billings_Ovulation_Method_-_Observation_Chart_-_en.svg" class="internal" title="Billings Ovulation Method - Observation Chart - en.svg"><b>This illustration</b></a> <b>was made by</b> </i><b><a href="//commons.wikimedia.org/wiki/User:Peter17" title="User:Peter17">Peter Potrowl</a></b><i>.</i>
<p>Please credit this with : <b>© Peter Potrowl</b> in the immediate vicinity of the image. A link to my website <a rel="nofollow" class="external text" href="https://www.sitemai.eu">sitemai.eu</a> would be much appreciated but isn't mandatory.
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<a href="//commons.wikimedia.org/wiki/File:UserIconMail.svg" class="image"><img alt="UserIconMail.svg" src="https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/UserIconMail.svg/24px-UserIconMail.svg.png" decoding="async" width="24" height="24" srcset="https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/UserIconMail.svg/36px-UserIconMail.svg.png 1.5x, https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/UserIconMail.svg/48px-UserIconMail.svg.png 2x" data-file-width="700" data-file-height="700"></a>
<a href="//commons.wikimedia.org/wiki/Special:EmailUser/Peter17" title="Special:EmailUser/Peter17">Peter Potrowl</a> would be appreciated too.
</p><p>Do not copy this image illegally by ignoring the terms of the license below, as it is <b>not</b> in the public domain.
If you would like special permission to use, license, or purchase the image please contact me <a href="//commons.wikimedia.org/wiki/File:UserIconMail.svg" class="image"><img alt="UserIconMail.svg" src="https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/UserIconMail.svg/24px-UserIconMail.svg.png" decoding="async" width="24" height="24" srcset="https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/UserIconMail.svg/36px-UserIconMail.svg.png 1.5x, https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/UserIconMail.svg/48px-UserIconMail.svg.png 2x" data-file-width="700" data-file-height="700"></a><a href="//commons.wikimedia.org/wiki/Special:EmailUser/Peter17" title="Special:EmailUser/Peter17">Peter Potrowl</a> to negotiate terms.
</p><p>More free pictures in my <a rel="nofollow" class="external text" href="https://www.sitemai.eu">website</a>. Donations are accepted <a rel="nofollow" class="external text" href="https://paypal.me/Peter017">here</a> and <a rel="nofollow" class="external text" href="https://buymeacoffee.com/peter17">here</a>.
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<span class="int-own-work" lang="en">Own work</span>