diff --git a/python-package/lets_plot/plot/core.py b/python-package/lets_plot/plot/core.py
index bbba48055ff..9cbfcde5527 100644
--- a/python-package/lets_plot/plot/core.py
+++ b/python-package/lets_plot/plot/core.py
@@ -130,6 +130,10 @@ def layer(geom=None, stat=None, data=None, mapping=None, position=None, **kwargs
     return LayerSpec(**locals())
 
 
+def _filter_none(original: dict) -> dict:
+    return {k: v for k, v in original.items() if v is not None}
+
+
 #
 #  -----------------------------------
 #  Specs
@@ -145,7 +149,7 @@ def _specs_to_dict(opts_raw):
         else:
             opts[k] = v
 
-    return opts
+    return _filter_none(opts)
 
 
 class FeatureSpec():
diff --git a/python-package/lets_plot/plot/tooltip.py b/python-package/lets_plot/plot/tooltip.py
index 55bc9ebf131..d023018b507 100644
--- a/python-package/lets_plot/plot/tooltip.py
+++ b/python-package/lets_plot/plot/tooltip.py
@@ -2,9 +2,10 @@
 #  Copyright (c) 2020. JetBrains s.r.o.
 #  Use of this source code is governed by the MIT license that can be found in the LICENSE file.
 
-from .core import FeatureSpec
 from typing import List
 
+from lets_plot.plot.core import FeatureSpec, _filter_none
+
 #
 # Tooltips
 #
@@ -115,7 +116,7 @@ def as_dict(self):
         d['tooltip_min_width'] = self._tooltip_min_width
         d['tooltip_color'] = self._tooltip_color
         d['tooltip_variables'] = self._tooltip_variables
-        return d
+        return _filter_none(d)
 
     def format(self, field=None, format=None):
         """
diff --git a/python-package/test/plot/test_aes.py b/python-package/test/plot/test_aes.py
index 10aa435b5c6..f234a718e70 100644
--- a/python-package/test/plot/test_aes.py
+++ b/python-package/test/plot/test_aes.py
@@ -8,7 +8,7 @@
 
 
 class TestWithListArgs:
-    result_empty = {'x': None, 'y': None}
+    result_empty = {}
     result_xy = {'x': 'xVar', 'y': 'yVar'}
 
     @pytest.mark.parametrize('args,expected', [
diff --git a/python-package/test/plot/test_assembly.py b/python-package/test/plot/test_assembly.py
index 2efa857abcc..93356b381bc 100644
--- a/python-package/test/plot/test_assembly.py
+++ b/python-package/test/plot/test_assembly.py
@@ -17,14 +17,14 @@ def test_plot_geom_geom():
 
     expect = {
         'kind': 'plot',
-        'data': None,
+        'mapping': {},
         'data_meta': {},
-        'mapping': {'x': None, 'y': None},
         'layers': [
             geom1.as_dict(),
             geom2.as_dict()
         ],
-        'scales': []}
+        'scales': []
+    }
 
     assert (plot + geom1 + geom2).as_dict() == expect
     assert (plot + (geom1 + geom2)).as_dict() == expect
@@ -38,15 +38,11 @@ def test_plot_geom_scale():
 
     expect = {
         'kind': 'plot',
-        'data': None,
+        'mapping': {},
         'data_meta': {},
-        'mapping': {'x': None, 'y': None},
-        'layers': [
-            geom.as_dict()
-        ],
-        'scales': [
-            scale.as_dict()
-        ]}
+        'layers': [geom.as_dict()],
+        'scales': [scale.as_dict()]
+    }
 
     assert (plot + geom + scale).as_dict() == expect
     assert (plot + scale + geom).as_dict() == expect
@@ -58,13 +54,12 @@ def test_plot_ggtitle():
     ggtitle = gg.labs(title="New plot title")
 
     expect = {
+        'ggtitle': {'text': 'New plot title'},
         'kind': 'plot',
-        'data': None,
-        'mapping': {'x': None, 'y': None},
         'data_meta': {},
         'layers': [],
-        'scales': [],
-        'ggtitle': {'text': 'New plot title'}
+        'mapping': {},
+        'scales': []
     }
 
     assert (plot + ggtitle).as_dict() == expect
diff --git a/python-package/test/plot/test_geom.py b/python-package/test/plot/test_geom.py
index 2176fa60b9b..f9f385bdd06 100644
--- a/python-package/test/plot/test_geom.py
+++ b/python-package/test/plot/test_geom.py
@@ -16,27 +16,15 @@ class TestWithListAndDictArgs:
     mapping_arg = gg.aes('X')
     expected[0] = dict(
         geom='n',
-        mapping=mapping_arg.as_dict(),
-        data=None,
-        stat=None,
-        position=None,
-        show_legend=None,
-        sampling=None,
-        tooltips=None,
-        data_meta= {},
+        data_meta={},
+        mapping=mapping_arg.as_dict()
     )
 
     # II
     expected[1] = dict(
         geom='n',
-        mapping={'x': None, 'y': None},
-        data=None,
-        stat=None,
-        position=None,
-        show_legend=None,
-        sampling=None,
-        tooltips=None,
-        data_meta= {},
+        mapping={},
+        data_meta={},
         arrow={'angle': 0, 'length': 1, 'ends': 'a', 'type': 'b', 'name': 'arrow'}
     )
 
diff --git a/python-package/test/plot/test_geom_image.py b/python-package/test/plot/test_geom_image.py
index 6bcb27253f8..a87183a0dd7 100644
--- a/python-package/test/plot/test_geom_image.py
+++ b/python-package/test/plot/test_geom_image.py
@@ -12,6 +12,7 @@
 
 def _image_spec(width, height, href):
     return dict(
+        data_meta={},
         geom='image',
         # image_spec=dict(
         #     width=width,
@@ -20,21 +21,12 @@ def _image_spec(width, height, href):
         #     bytes=bytes
         # ),
         href=href,
-        data=None,
         mapping=dict(
-            x=None,
-            y=None,
             xmin=[-0.5],
             ymin=[-0.5],
             xmax=[width - 1 + 0.5],
             ymax=[height - 1 + 0.5],
-        ),
-        stat=None,
-        position=None,
-        show_legend=None,
-        data_meta={},
-        sampling=None,
-        tooltips=None,
+        )
     )
 
 
diff --git a/python-package/test/plot/test_geom_livemap.py b/python-package/test/plot/test_geom_livemap.py
new file mode 100644
index 00000000000..19eff4ea040
--- /dev/null
+++ b/python-package/test/plot/test_geom_livemap.py
@@ -0,0 +1,18 @@
+#  Copyright (c) 2021. JetBrains s.r.o.
+#  Use of this source code is governed by the MIT license that can be found in the LICENSE file.
+import pytest
+
+import lets_plot as gg
+
+
+@pytest.mark.parametrize('args,expected', [
+    ('foo', [['foo'], None]),
+    (['foo'], [['foo'], None]),
+    (['foo', 'bar'], [['foo'], ['bar']]),
+    ([['foo', 'bar']], [['foo', 'bar'], None]),
+    ([['foo', 'bar'], ['baz', 'qux']], [['foo', 'bar'], ['baz', 'qux']])
+])
+def test_map_join(args, expected):
+    # ggplot is required - it normalizes map_join on before_append
+    spec = gg.ggplot() + gg.geom_livemap(map_join=args, tooltips=gg.layer_tooltips())
+    assert spec.as_dict()['layers'][0]['map_join'] == expected
diff --git a/python-package/test/plot/test_ggplot.py b/python-package/test/plot/test_ggplot.py
index 51341304386..cf0c7eba0a6 100644
--- a/python-package/test/plot/test_ggplot.py
+++ b/python-package/test/plot/test_ggplot.py
@@ -6,15 +6,15 @@
 
 import lets_plot as gg
 
-data = [1, 2]
+data = {'a': [1, 2], 'b': [3, 4]}
 mapping_empty = gg.aes()
 mapping_x = gg.aes('X')
 
-result_empty = {'data': None, 'data_meta': {}, 'mapping': {'x': None, 'y': None}, 'layers': [], 'scales': [], 'kind': 'plot'}
-result_data = {'data': data, 'data_meta': {}, 'mapping': {'x': None, 'y': None}, 'layers': [], 'scales': [], 'kind': 'plot'}
-result_data_mapping_empty = {'data': data, 'data_meta': {}, 'mapping': mapping_empty.as_dict(), 'layers': [], 'scales': [], 'kind': 'plot'}
-result_data_mapping_x = {'data': data, 'data_meta': {}, 'mapping': mapping_x.as_dict(), 'layers': [], 'scales': [], 'kind': 'plot'}
-result_mapping_x = {'data': None, 'data_meta': {}, 'mapping': mapping_x.as_dict(), 'layers': [], 'scales': [], 'kind': 'plot'}
+result_empty = {'kind': 'plot', 'mapping': {}, 'data_meta': {}, 'layers': [], 'scales': []}
+result_data = {'data': data, 'kind': 'plot', 'mapping': {}, 'data_meta': {}, 'layers': [], 'scales': []}
+result_data_mapping_empty = {'data': data, 'kind': 'plot', 'mapping': {}, 'data_meta': {}, 'layers': [], 'scales': []}
+result_data_mapping_x = {'data': data, 'mapping': {'x': 'X'}, 'data_meta': {}, 'kind': 'plot', 'layers': [], 'scales': []}
+result_mapping_x = {'mapping': {'x': 'X'}, 'data_meta': {}, 'kind': 'plot', 'layers': [], 'scales': []}
 
 
 @pytest.mark.parametrize('args,expected', [
diff --git a/python-package/test/plot/test_ggsize.py b/python-package/test/plot/test_ggsize.py
index bec2c38f585..b31ce2bda69 100644
--- a/python-package/test/plot/test_ggsize.py
+++ b/python-package/test/plot/test_ggsize.py
@@ -9,10 +9,4 @@
 # noinspection SpellCheckingInspection
 def test_ggsize():
     spec = gg.ggplot() + gg.ggsize(5, 10)
-    assert spec.as_dict() == {'data': None,
-                              'data_meta': {},
-                              'mapping': {'x': None, 'y': None},
-                              'kind': 'plot',
-                              'ggsize': {'height': 10, 'width': 5},
-                              'layers': [],
-                              'scales': []}
+    assert spec.as_dict() == {'kind': 'plot', 'ggsize': {'height': 10, 'width': 5}, 'mapping': {}, 'data_meta': {}, 'layers': [], 'scales': []}
diff --git a/python-package/test/plot/test_scale.py b/python-package/test/plot/test_scale.py
index 5937eb28d19..366f29de513 100644
--- a/python-package/test/plot/test_scale.py
+++ b/python-package/test/plot/test_scale.py
@@ -13,12 +13,9 @@
 def gen_scale_args():
     pos_args = ['a', 'n']  # aesthetic
     pos_args_as_dict = {'aesthetic': 'a', 'name': 'n'}
-    pos_args_def_as_dict = {'breaks': None, 'labels': None, 'limits': None, 'expand': None, 'na_value': None,
-                            'guide': None, 'trans': None, 'format': None}
     other_args = {'other1': 1, 'other2': 2}
 
     expected = pos_args_as_dict.copy()
-    expected.update(pos_args_def_as_dict)
     expected.update(other_args)
     return pos_args, other_args, expected
 
diff --git a/python-package/test/plot/test_theme.py b/python-package/test/plot/test_theme.py
index d4be977bc1c..05712c9d340 100644
--- a/python-package/test/plot/test_theme.py
+++ b/python-package/test/plot/test_theme.py
@@ -6,7 +6,7 @@
 from lets_plot import element_rect
 from lets_plot.plot import theme
 from lets_plot.plot.geom import _geom
-from lets_plot.plot.theme_classic_ import theme_classic
+from lets_plot.plot.theme_set import theme_classic
 
 
 def test_theme_options_should_be_merged():