Update tests for the /plot/<metric> route:

+ separate out test_plot_with_data to '_by_name' and '_by_id'
+ Remove skipped test - replaced by new ones. (which are also skipped...
  but now I have a better reason for it!)

tests/test_routes.py

@@ -56,22 +56,28 @@ def test_index_with_data(client, populated_db):
     assert b"foo.bar" in rv.data
 
 
-@pytest.mark.xfail(reason="Needs code updates")
-def test_plot(client):
-    rv = client.get("/plot/foo")
-    assert rv.status_code == 404
-
-
-def test_plot_with_data(client, populated_db):
+@pytest.mark.xfail(
+    reason="JS callbacks are async: plotly div not populated immediatly"
+)
+def test_plot_with_data_by_name(client, populated_db):
     rv = client.get("/plot/foo")
     assert rv.status_code == 200
     assert b"foo" in rv.data
     assert b'<div id="graph"' in rv.data
+    # TODO: make this work. Issue is JS callbacks.
+    #  assert b'<div class="plot-container plotly">' in rv.data
+
 
+@pytest.mark.xfail(
+    reason="JS callbacks are async: plotly div not populated immediatly"
+)
+def test_plot_with_data_by_id(client, populated_db):
     rv = client.get("/plot/1")
     assert rv.status_code == 200
     assert b"foo" in rv.data
     assert b'<div id="graph"' in rv.data
+    # TODO: make this work. Issue is JS callbacks.
+    #  assert b'<div class="plot-container plotly">' in rv.data
 
 
 def test_api_add(client):