title: How to make exploratory plots using only certain rows in a column
body: <p>I am making some exploratory plots to analyze zone M. I need one that plots Distance over time and another with Distance vs. MHT.</p>
  <p>Here is what I have so far:</p>
  <pre><code>library(ggplot2)
  
  ggplot(datmarsh, aes(x=Year, y=Distance)) + geom_point()
  ggplot(datmarsh, aes(x=MHT, y=Distance)) + geom_point()
  </code></pre>
  <p>What I'm struggling with is specifying only zone &quot;M&quot; in each of these graphs.</p>
  <p>Here is a sample of what my data looks like:</p>
  <pre><code>Year    Distance    MHT Zone
  1975    253.1875    933 M
  1976    229.75      877 M
  1977    243.8125    963 M
  1978    243.8125    957 M
  1975    103.5       933 P
  1976    150.375     877 P
  1977    117.5625    963 P
  1978    131.625     957 P
  1979    145.6875    967 P
  1975    234.5       933 PP
  1976    314.1875    877 PP
  1977    248.5625    963 PP
  1978    272         957 PP
  1979    290.75      967 PP
  </code></pre>
  <p>Thanks!</p>