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<p class="MsoNormal"><span lang="EN-US">The fixed step differentiation plugin, like all of the filter plugins, assumes a constant sample rate (which, in this case, is a parameter in the plugin).  </span>In other words, it probably isn't what you want :-(<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">One could ~easily write a plugin which would take both X and Y and do what you want....  Use one of the fit plugins as a template.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hey, there’s a much easier way to achieve that than writing your own plugin:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo2"><![if !supportLists]><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">1)<span style="font:7.0pt "Times New Roman"">     
</span></span></span><![endif]><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Generate a regularly-spaced time vector (Create -> Vector, then Generate and select the appropriate min, max and number of points)<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo2"><![if !supportLists]><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">2)<span style="font:7.0pt "Times New Roman"">     
</span></span></span><![endif]><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Use an interpolation plugin: Create -> Standard Plugin -> Interpolation [xxx], use the previous regular time vector as X’ vector<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span style="mso-list:Ignore">3)<span style="font:7.0pt "Times New Roman"">     
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Do the differentiation on the interpolated, regularly-sampled data.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">@Barth: I tried using [Time:Min (X2)] and [Time:Max (X1)] as bounds in the fields of the Generate vector dialog and it does not seem to work. I
 thought it did, have I been dreaming?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Nicolas
<o:p></o:p></span></p>
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