Histogram by levels
<p>My data consists in four time series (here, from A to D), and I want to observe the frequency of a continuous value within each serie, through time.
That is, I would like to have time on the x-axis, with A, B, C, and D, and, for each time range, a vertical histogram.</p>

<p>To be more clear, I would like to obtain something similar to </p>

<p><img src="https://i.stack.imgur.com/qpS2a.jpg" alt="what I get using this code">:</p>

<pre><code>ggplot(data, aes(x=time, y=values)) + geom_boxplot()
</code></pre>

<p>but with histograms instead of boxplots.</p>

<p>Here is the data:</p>

<pre><code>values  time
0,24    A
0,70    A
0,36    A
0,73    A
0,72    A
0,72    A
0,58    A
0,62    A
0,24    B
0,76    B
0,70    B
0,36    B
0,73    B
0,62    B
0,24    C
0,63    C
0,71    C
0,36    C
0,73    C
0,92    C
0,62    C
0,85    C
0,40    C
0,49    C
0,63    D
0,73    D
0,71    D
0,92    D
0,57    D
0,92    D
0,85    D
</code></pre>