diff --git a/pandas/_libs/tslibs/frequencies.pxd b/pandas/_libs/tslibs/frequencies.pxd
index c96661aaab443..09397a02c5ea0 100644
--- a/pandas/_libs/tslibs/frequencies.pxd
+++ b/pandas/_libs/tslibs/frequencies.pxd
@@ -3,6 +3,5 @@ cdef dict attrname_to_abbrevs
 cpdef str get_rule_month(object source, str default=*)
 
 cpdef get_freq_code(freqstr)
-cpdef str get_base_alias(freqstr)
 cpdef int get_to_timestamp_base(int base)
 cpdef str get_freq_str(base, mult=*)
diff --git a/pandas/_libs/tslibs/frequencies.pyx b/pandas/_libs/tslibs/frequencies.pyx
index 60f750da92091..ad00299bb9b25 100644
--- a/pandas/_libs/tslibs/frequencies.pyx
+++ b/pandas/_libs/tslibs/frequencies.pyx
@@ -297,21 +297,6 @@ cpdef str get_freq_str(base, mult=1):
     return str(mult) + code
 
 
-cpdef str get_base_alias(freqstr):
-    """
-    Returns the base frequency alias, e.g., '5D' -> 'D'
-
-    Parameters
-    ----------
-    freqstr : str
-
-    Returns
-    -------
-    base_alias : str
-    """
-    return base_and_stride(freqstr)[0]
-
-
 cpdef int get_to_timestamp_base(int base):
     """
     Return frequency code group used for base of to_timestamp against
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index f1f8abb9e93a2..8b2925b2c0827 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -665,7 +665,7 @@ def _addsub_int_array(
         res_values[self._isnan] = iNaT
         return type(self)(res_values, freq=self.freq)
 
-    def _add_offset(self, other):
+    def _add_offset(self, other: DateOffset):
         assert not isinstance(other, Tick)
 
         if other.base != self.freq.base:
diff --git a/pandas/plotting/_matplotlib/timeseries.py b/pandas/plotting/_matplotlib/timeseries.py
index f6e120e2f91e7..f62340ee8f756 100644
--- a/pandas/plotting/_matplotlib/timeseries.py
+++ b/pandas/plotting/_matplotlib/timeseries.py
@@ -1,12 +1,13 @@
 # TODO: Use the fact that axis can have units to simplify the process
 
 import functools
+from typing import Optional
 
 import numpy as np
 
 from pandas._libs.tslibs.frequencies import (
     FreqGroup,
-    get_base_alias,
+    base_and_stride,
     get_freq_code,
     is_subperiod,
     is_superperiod,
@@ -165,6 +166,16 @@ def _get_ax_freq(ax):
     return ax_freq
 
 
+def get_period_alias(freq) -> Optional[str]:
+    if isinstance(freq, DateOffset):
+        freq = freq.rule_code
+    else:
+        freq = base_and_stride(freq)[0]
+
+    freq = frequencies.get_period_alias(freq)
+    return freq
+
+
 def _get_freq(ax, series):
     # get frequency from data
     freq = getattr(series.index, "freq", None)
@@ -178,12 +189,7 @@ def _get_freq(ax, series):
         freq = ax_freq
 
     # get the period frequency
-    if isinstance(freq, DateOffset):
-        freq = freq.rule_code
-    else:
-        freq = get_base_alias(freq)
-
-    freq = frequencies.get_period_alias(freq)
+    freq = get_period_alias(freq)
     return freq, ax_freq
 
 
@@ -200,11 +206,7 @@ def _use_dynamic_x(ax, data):
     if freq is None:
         return False
 
-    if isinstance(freq, DateOffset):
-        freq = freq.rule_code
-    else:
-        freq = get_base_alias(freq)
-    freq = frequencies.get_period_alias(freq)
+    freq = get_period_alias(freq)
 
     if freq is None:
         return False
@@ -238,8 +240,6 @@ def _maybe_convert_index(ax, data):
 
         if freq is None:
             freq = getattr(data.index, "inferred_freq", None)
-        if isinstance(freq, DateOffset):
-            freq = freq.rule_code
 
         if freq is None:
             freq = _get_ax_freq(ax)
@@ -247,8 +247,7 @@ def _maybe_convert_index(ax, data):
         if freq is None:
             raise ValueError("Could not get frequency alias for plotting")
 
-        freq = get_base_alias(freq)
-        freq = frequencies.get_period_alias(freq)
+        freq = get_period_alias(freq)
 
         if isinstance(data.index, ABCDatetimeIndex):
             data = data.tz_localize(None).to_period(freq=freq)