I have a function which manipulates a ggplot object, by converting it to a grob and then modifying the layers. I would like the function to return a ggplot object not a grob. Is there a simple way to convert a grob back to gg? 

[The documentation][1] on `ggplotGrob` is awfully sparse.  
Simple example:

    P &lt;- ggplot(iris) + geom_bar(aes(x=Species, y=Petal.Width), stat=&quot;identity&quot;)

    G &lt;- ggplotGrob(P)
    ... some manipulation to G ...

    ## DESIRED: 
    P2 &lt;- inverse_of_ggplotGrob(G)

    such that, we can continue to use basic ggplot syntax, ie
    `P2 + ylab (&quot;The Width of the Petal&quot;)`


  [1]: http://docs.ggplot2.org/0.9.3/ggplotGrob.html


---- 
UPDATE: 

To answer the question in the comment, the motivation here is to modify the colors of facet labels programmatically, based on the value of label name in each facet.  The functions below work nicely (based on input from baptise in a previous question).  

I would like for the return value from `colorByGroup` to be a ggplot object, not simply a grob. 

Here is the code, for those interested

    get_grob_strips &lt;- function(G, strips=grep(pattern=&quot;strip.*&quot;, G$layout$name)) {

      if (inherits(G, &quot;gg&quot;))
        G &lt;- ggplotGrob(G)
      if (!inherits(G, &quot;gtable&quot;))
        stop (&quot;G must be a gtable object or a gg object&quot;)

      strip.type &lt;- G$layout[strips, &quot;name&quot;]
      ## I know this works for a simple 
      strip.nms &lt;- sapply(strips, function(i) {
         attributes(G$grobs[[i]]$width$arg1)$data[[1]][[&quot;label&quot;]]
      })

      data.table(grob_index=strips, type=strip.type, group=strip.nms)
    }


    refill &lt;- function(strip, colour){
      strip[[&quot;children&quot;]][[1]][[&quot;gp&quot;]][[&quot;fill&quot;]] &lt;- colour
      return(strip)
    }

    colorByGroup &lt;- function(P, colors, showWarnings=TRUE) {
    ## The names of colors should match to the groups in facet
      G &lt;- ggplotGrob(P)
      DT.strips &lt;- get_grob_strips(G)

      groups &lt;- names(colors)
      if (is.null(groups) || !is.character(groups)) {
        groups &lt;- unique(DT.strips$group)
        if (length(colors) &lt; length(groups))
          stop (&quot;not enough colors specified&quot;)
        colors &lt;- colors[seq(groups)]
        names(colors) &lt;- groups
      }


      ## &#39;groups&#39; should match the &#39;group&#39; in DT.strips, which came from the facet_name
      matched_groups &lt;- intersect(groups, DT.strips$group)
      if (!length(matched_groups))
        stop (&quot;no groups match&quot;)
      if (showWarnings) {
          if (length(wh &lt;- setdiff(groups, DT.strips$group)))
            warning (&quot;values in &#39;groups&#39; but not a facet label: \n&quot;, paste(wh, colapse=&quot;, &quot;))
          if (length(wh &lt;- setdiff(DT.strips$group, groups)))
            warning (&quot;values in facet label but not in &#39;groups&#39;: \n&quot;, paste(wh, colapse=&quot;, &quot;))
      }

      ## identify the indecies to the grob and the appropriate color
      DT.strips[, color := colors[group]]
      inds &lt;- DT.strips[!is.na(color), grob_index]
      cols &lt;- DT.strips[!is.na(color), color]

      ## Fill in the appropriate colors, using refill()
      G$grobs[inds] &lt;- mapply(refill, strip = G$grobs[inds], colour = cols, SIMPLIFY = FALSE)

      G
    }

 



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Inverse of ggplotGrob?