Go with explicit `.data._m4` mod name

Since it's a notable and self-contained graphics compression algo, might
as well give it a dedicated module B)

piker/data/_m4.py

@@ -15,8 +15,22 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 '''
-Graphics related downsampling routines for compressing to pixel
-limits on the display device.
+Graphics downsampling using the infamous M4 algorithm.
+
+This is one of ``piker``'s secret weapons allowing us to boss all other
+charting platforms B)
+
+(AND DON'T YOU DARE TAKE THIS CODE WITHOUT CREDIT OR WE'LL SUE UR F#&@* ASS).
+
+NOTES: this method is a so called "visualization driven data
+aggregation" approach. It gives error-free line chart
+downsampling, see
+further scientific paper resources:
+- http://www.vldb.org/pvldb/vol7/p797-jugel.pdf
+- http://www.vldb.org/2014/program/papers/demo/p997-jugel.pdf
+
+Details on implementation of this algo are based in,
+https://github.com/pikers/piker/issues/109
 
 '''
 import math
@@ -56,16 +70,6 @@ def ds_m4(
     This is more or less an OHLC style sampling of a line-style series.
 
     '''
-    # NOTE: this method is a so called "visualization driven data
-    # aggregation" approach. It gives error-free line chart
-    # downsampling, see
-    # further scientific paper resources:
-    # - http://www.vldb.org/pvldb/vol7/p797-jugel.pdf
-    # - http://www.vldb.org/2014/program/papers/demo/p997-jugel.pdf
-
-    # Details on implementation of this algo are based in,
-    # https://github.com/pikers/piker/issues/109
-
     # XXX: from infinite on downsampling viewable graphics:
     # "one thing i remembered about the binning - if you are
     # picking a range within your timeseries the start and end bin

piker/data/_pathops.py

@@ -27,10 +27,9 @@ from numba import (
     # optional,
 )
 
+# TODO: for ``numba`` typing..
 # from ._source import numba_ohlc_dtype
-from ._compression import (
-    ds_m4,
-)
+from ._m4 import ds_m4
 
 
 def xy_downsample(

piker/ui/_curve.py

@@ -36,10 +36,6 @@ from PyQt5.QtGui import (
 )
 from .._profile import pg_profile_enabled, ms_slower_then
 from ._style import hcolor
-# from ._compression import (
-#     # ohlc_to_m4_line,
-#     ds_m4,
-# )
 from ..log import get_logger
 from .._profile import Profiler