Reading Min and Max From Text File
The value of atof("Volt") is zero.  The corresponding values of atof("Time") and atof("Ampere") are also zero, which gives you the minimum values of zero you observed.  

You need to skip the first line of the file.  Adding this before the while loop would work:

myFile.ignore(80, '\n');


Use std::numeric_limits&lt;std::streamsize&gt;::max() instead of 80 for a more correct (but verbose) limit of characters to skip before the new line character. 



After you add the skipping of the first line, you no longer need the a, b &amp; c variables and atof() calls;  you can read the values directly:

while (myFile &gt;&gt; time &gt;&gt; volt &gt;&gt; ampere)