View a graph.

def view_graph(graph_str, parent=None, prune_to=None):
    """View a graph."""
    from rezgui.dialogs.ImageViewerDialog import ImageViewerDialog
    from rez.config import config

    # check for already written tempfile
    h = hash((graph_str, prune_to))
    filepath = graph_file_lookup.get(h)
    if filepath and not os.path.exists(filepath):
        filepath = None

    # write graph to tempfile
    if filepath is None:
        suffix = ".%s" % config.dot_image_format
        fd, filepath = tempfile.mkstemp(suffix=suffix, prefix="rez-graph-")
        os.close(fd)

        dlg = WriteGraphDialog(graph_str, filepath, parent, prune_to=prune_to)
        if not dlg.write_graph():
            return

    # display graph
    graph_file_lookup[h] = filepath
    dlg = ImageViewerDialog(filepath, parent)
    dlg.exec_()