title: How to call specific id's or variables in ggplot2?
body: <p>This is the edited question. I would like to create a multi-facet graph based on panel data. While graphing panel data with only one y-variable is relatively simple, I am asking how can I create a graph in R using multiple y-variables that should appear in the same graph.
  The problem is that I have two &quot;y&quot;. Each ggplot has the <code>(aes(x=year, y=something, ...</code> but I have two &quot;y's&quot;, namely source1 and source2 and I can't find a solution to create a multi-facet graph that contains both y-variables in the same facet. Please see the description of the panel data below.
  The panel data that I want to graph in R looks like this:
  <a href="https://i.stack.imgur.com/PzAMR.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/PzAMR.png" alt="![enter image description here" /></a></p>
  <p>Or in txt format:</p>
  <pre><code>id  area    element item    year    value   source1 source2 diff    relative
  46  Australia   Yield   Barley  2000    20080   2.02    1.67    0.35    17.32673267
  46  Australia   Yield   Barley  2010    18268   1.9 1.75    0.15    7.894736842
  46  Australia   Yield   Barley  2018    23044   2.46    1.86    0.6 24.3902439
  113 Australia   Yield   Maize   2000    58727   5.95    6.88    -0.93   -15.6302521
  113 Australia   Yield   Maize   2010    67515   6.97    7.54    -0.57   -8.177905308
  113 Australia   Yield   Maize   2018    82594   8   8.54    -0.54   -6.75
  238 Australia   Yield   Potatoes    2000    314873  33.74   7.33    26.4    78.27504446
  238 Australia   Yield   Potatoes    2010    383895  38.73   7.81    30.92   79.83475342
  238 Australia   Yield   Potatoes    2018    426258  42.39   8.56    33.82   79.80655815
  2224    Australia   Yield   Rapeseed    2000    12177   1.23    1.11    0.12    9.756097561
  2224    Australia   Yield   Rapeseed    2010    11261   1.12    1.23    -0.11   -9.821428571
  2224    Australia   Yield   Rapeseed    2018    12280   1.39    1.36    0.03    2.158273381
  5557    Australia   Yield   Soybeans    2000    18714   1.87    2.08    -0.21   -11.22994652
  5557    Australia   Yield   Soybeans    2010    19042   1.84    2.24    -0.4    -21.73913043
  5557    Australia   Yield   Soybeans    2018    17027   1.81    2.43    -0.62   -34.25414365
  757 Australia   Yield   SugC    2000    0   0   27.45   -27.45  #DIV/0!
  757 Australia   Yield   SugC    2010    0   0   28.76   -28.76  #DIV/0!
  757 Australia   Yield   SugC    2018    0   0   30.27   -30.27  #DIV/0!
  8890    Australia   Yield   Sunflower seed  2000    10494   1.08    1.02    0.06    5.555555556
  8890    Australia   Yield   Sunflower seed  2010    15414   1.25    1.13    0.12    9.6
  8890    Australia   Yield   Sunflower seed  2018    18235   1.32    1.26    0.06    4.545454545
  33335   Australia   Yield   Wheat   2000    18398   1.9 1.51    0.4 20.52631579
  33335   Australia   Yield   Wheat   2010    15987   1.75    1.73    0.02    1.142857143
  33335   Australia   Yield   Wheat   2018    19444   2.19    1.99    0.2 9.132420091
  48  Brazil  Yield   Barley  2000    19437   2.12    1.88    0.25    11.32075472
  48  Brazil  Yield   Barley  2010    33115   3.12    2.08    1.04    33.33333333
  48  Brazil  Yield   Barley  2018    32591   3.31    2.33    0.98    29.60725076
  115 Brazil  Yield   Maize   2000    27182   2.97    2.64    0.33    11.11111111
  115 Brazil  Yield   Maize   2010    43667   4.1 3.76    0.34    8.292682927
  115 Brazil  Yield   Maize   2018    51044   5   5.13    -0.13   -2.6
  240 Brazil  Yield   Potatoes    2000    171813  17.38   3.59    13.79   79.34407365
  240 Brazil  Yield   Potatoes    2010    258859  25.66   3.92    21.74   84.72330475
  240 Brazil  Yield   Potatoes    2018    311760  30.59   4.32    26.27   85.87773782
  2226    Brazil  Yield   Rapeseed    2000    17083   1.59    1.86    -0.26   -16.98113208
  2226    Brazil  Yield   Rapeseed    2010    15217   1.37    2.22    -0.84   -62.04379562
  2226    Brazil  Yield   Rapeseed    2018    12500   1.3 2.66    -1.36   -104.6153846
  5559    Brazil  Yield   Soybeans    2000    24033   2.5 2.28    0.22    8.8
  5559    Brazil  Yield   Soybeans    2010    29475   2.9 2.58    0.32    11.03448276
  5559    Brazil  Yield   Soybeans    2018    33903   3.22    2.95    0.28    8.385093168
  1478    Brazil  Yield   SugC    2000    0   0   15.01   -15.01  #DIV/0!
  1478    Brazil  Yield   SugC    2010    0   0   16.43   -16.43  #DIV/0!
  1478    Brazil  Yield   SugC    2018    0   0   18.2    -18.2   #DIV/0!
  8892    Brazil  Yield   Sunflower seed  2000    16724   1.53    1.55    -0.01   -1.307189542
  8892    Brazil  Yield   Sunflower seed  2010    11322   1.21    1.77    -0.57   -46.28099174
  8892    Brazil  Yield   Sunflower seed  2018    16064   1.55    2.05    -0.5    -32.25806452
  33337   Brazil  Yield   Wheat   2000    15156   1.81    1.43    0.38    20.99447514
  33337   Brazil  Yield   Wheat   2010    28288   2.52    1.94    0.59    23.01587302
  33337   Brazil  Yield   Wheat   2018    26238   2.69    2.56    0.13    
  ...and so on
  </code></pre>
  <p>I've created the panel data in R:</p>
  <pre><code>library(plm)
  panel &lt;- pdata.frame(data, index = c(&quot;id&quot;, &quot;year&quot;), drop.index = FALSE)
  </code></pre>
  <p>Then I've tried this plot in ggplot:</p>
  <pre><code>ggplot(df, aes(x=year, y=diff, group=area, fill=area)) +
      geom_area() +
      scale_fill_viridis(discrete = TRUE) +
      theme(legend.position=&quot;none&quot;) +
      ggtitle(&quot;Yield&quot;) +
      theme_ipsum() +
      theme(
        legend.position=&quot;none&quot;,
        panel.spacing = unit(0.1, &quot;lines&quot;),
        strip.text.x = element_text(size = 8),
        plot.title = element_text(size=14)
      ) +
      facet_wrap(~item)
  </code></pre>
  <p>however, it does not shows facets on area and then inside each facet the values of source1 and source2 values for each crop.
  I would like to create something like this:</p>
  <p><a href="https://i.stack.imgur.com/0pDs0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0pDs0.png" alt="enter image description here" /></a></p>
  <p>The problem becomes even more complicate because I have many dimensions: area, item, years and those two y-variables: source1 and source2.
  The most important is to create facets that show source1 and source2 appear as lines or bars for comparison reasons. But the problem is how to create facets that show this two y-variables by year, area and item? And all of these without producing an overcrowded graph.</p>
  <p>Because dc27 asked examples of graph, another possible example would be:
  <a href="https://i.stack.imgur.com/7vbDs.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7vbDs.png" alt="enter image description here" /></a></p>
  <p>where source1 and source2 should be 2 bars side by side showing values per year, per item and per area.
  If you have any other suggestions on how to plot this panel data you are more than welcome.</p>