title: Cross-column summary after `summary_rows` with gt
body: <p>This is a toy example, so the numbers are meaningless, but how would I calculate the summary statistics for <code>proportion</code> in the <code>summary_rows</code> row of the table per group?</p>
  <p><code>proportion</code> is a row-wise calculation, so I can't sum/mean/sd/etc. For <code>average proportion</code>, for example, I want to calculate <code>num</code>[average] / <code>items</code>[average]. I can't figure out how to get a custom function to work for <code>fns</code> across groups.</p>
  <pre><code>exibble_a &lt;-
    exibble %&gt;%
    mutate(items = runif(8,10,20)) %&gt;%
    group_by(group) %&gt;%
    mutate(proportion = ifelse(is.na(items / num), 0, items / num)) %&gt;%
    ungroup() %&gt;%
    select(-c(fctr, date, time, datetime))
  
  exibble_b = 
    exibble_a %&gt;%
    group_by(group) %&gt;%
    gt(rowname_col = &quot;row&quot;, groupname_col = &quot;group&quot;) %&gt;%
    fmt_missing(columns = everything()) %&gt;%
    fmt_percent(columns = proportion,
                decimals = 2) %&gt;%
    summary_rows(groups = TRUE,
                 columns = c(num,items),
                 fns = list(
                   average = ~ mean(.,na.rm=TRUE),
                   total = ~ sum(.,na.rm=TRUE),
                   SD = ~ sd(.,na.rm=TRUE))) %&gt;%
    summary_rows(groups = TRUE,
                 columns = proportion,
                 fns = list(
                   average = ~ mean(exibble_a$items) / mean(exibble_a$num, 
                                                            na.rm=TRUE)),
                 formatter = fmt_percent,
                 decimals = 2,
                 use_seps = TRUE)
  
  exibble_b
  </code></pre>