title: Plot logical data as rows of filled versus empty squares in R
body: <p>I'm tracking the habits I want to build in a text file / spreadsheet / whatever. It looks like this:</p>
  
  <pre><code>habits &lt;- read.table(textConnection("
  date study exercise clean
  2019-01-08 TRUE TRUE TRUE
  2019-01-09 FALSE FALSE TRUE
  2019-01-10 TRUE TRUE TRUE
  2019-01-11 FALSE TRUE TRUE
  2019-01-12 TRUE FALSE TRUE
  2019-01-13 FALSE TRUE TRUE
  2019-01-14 TRUE TRUE TRUE
  2019-01-15 FALSE FALSE TRUE
  2019-01-16 FALSE FALSE TRUE
  2019-01-17 FALSE FALSE TRUE
  2019-01-18 FALSE FALSE TRUE
  2019-01-19 FALSE FALSE TRUE
  2019-01-20 FALSE TRUE TRUE
  2019-01-21 FALSE FALSE TRUE
  2019-01-22 TRUE FALSE TRUE
  2019-01-23 FALSE TRUE TRUE
  2019-01-24 TRUE FALSE TRUE
  2019-01-25 FALSE FALSE TRUE
  2019-01-26 TRUE FALSE TRUE
  2019-01-27 FALSE FALSE TRUE
  2019-01-28 TRUE TRUE FALSE
  2019-01-29 FALSE TRUE TRUE
  2019-01-30 TRUE TRUE TRUE
  2019-01-31 FALSE TRUE TRUE
  2019-02-01 TRUE FALSE TRUE
  2019-02-02 FALSE TRUE TRUE
  2019-02-03 TRUE TRUE TRUE
  2019-02-04 FALSE FALSE TRUE
  2019-02-05 FALSE FALSE TRUE
  2019-02-06 FALSE TRUE TRUE
  2019-02-07 TRUE FALSE TRUE
  2019-02-08 FALSE FALSE TRUE
  2019-02-09 FALSE FALSE TRUE
  2019-02-10 TRUE TRUE TRUE
  2019-02-11 FALSE FALSE TRUE
  2019-02-12 FALSE TRUE TRUE
  2019-02-13 FALSE FALSE TRUE
  2019-02-14 FALSE FALSE FALSE
  2019-02-15 FALSE FALSE FALSE
  2019-02-16 FALSE FALSE FALSE
  2019-02-17 TRUE FALSE TRUE
  2019-02-18 FALSE FALSE TRUE
  2019-02-19 FALSE FALSE TRUE
  2019-02-20 FALSE FALSE TRUE
  2019-02-21 FALSE FALSE TRUE
  2019-02-22 TRUE FALSE TRUE
  2019-02-23 FALSE FALSE FALSE
  2019-02-24 FALSE FALSE FALSE
  2019-02-25 FALSE FALSE FALSE
  2019-02-26 FALSE FALSE FALSE
  2019-02-27 FALSE FALSE FALSE
  2019-02-28 FALSE FALSE FALSE
  "), header = TRUE, colClasses=c("Date", "logical", "logical", "logical"))
  </code></pre>
  
  <p>I'd like to visualize my adherence as a "habit tracker":</p>
  
  <p><a href="https://i.stack.imgur.com/aQH99.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/aQH99.jpg" alt="image of a hand-drawn habit tracker"></a></p>
  
  <p><strong>How do I plot logical data as rows of filled versus empty squares?</strong></p>
  
  <p><em>I would prefer a solution using base R, but that is not a must.</em></p>