diff --git a/.gitignore b/.gitignore
index bf3055e..49a04be 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,3 +10,4 @@ build/
 .pytest_cache/
 venv/
 .idea/
+.tox/
diff --git a/Makefile b/Makefile
index 93f5520..3681ab9 100644
--- a/Makefile
+++ b/Makefile
@@ -17,7 +17,7 @@ publish: tag upload
 
 clean:
 	@find . | grep -E "(__pycache__|\.pyc|\.pyo$\)" | xargs rm -rf
-	@rm -rf *.egg-info/ build/ dist/ MANIFEST .pytest_cache/
+	@rm -rf src/*.egg-info/ build/ dist/ MANIFEST .pytest_cache/ .tox/
 
 lint:
 	black --check .
diff --git a/setup.cfg b/setup.cfg
index 93c9daf..2025097 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,9 +1,9 @@
 [metadata]
 name = termplotlib
-version = 0.3.4
+version = 0.3.5
 author = Nico Schlömer
 author_email = nico.schloemer@gmail.com
-description = Plotting on the command line
+description = Python plotting for the command line
 url = https://github.com/nschloe/termplotlib
 project_urls =
     Code=https://github.com/nschloe/termplotlib
@@ -39,6 +39,7 @@ package_dir =
 packages = find:
 install_requires =
     importlib_metadata;python_version<"3.8"
+    numpy
 python_requires = >=3.6
 
 [options.packages.find]
diff --git a/src/termplotlib/barh.py b/src/termplotlib/barh.py
index d137eb7..f7a7f89 100644
--- a/src/termplotlib/barh.py
+++ b/src/termplotlib/barh.py
@@ -1,4 +1,6 @@
-from typing import List
+from typing import List, Optional
+
+import numpy as np
 
 from .helpers import is_unicode_standard_output
 
@@ -13,7 +15,12 @@ def _trim_trailing_zeros(lst):
 
 
 def barh(
-    vals, labels=None, max_width=40, bar_width=1, show_vals=True, force_ascii=False
+    vals: List[int],
+    labels: Optional[List[str]] = None,
+    max_width: int = 40,
+    bar_width: int = 1,
+    show_vals: bool = True,
+    force_ascii: bool = False,
 ):
     matrix = _get_matrix_of_eighths(vals, max_width, bar_width)
 
@@ -24,13 +31,14 @@ def barh(
 
     fmt = []
     if labels is not None:
-        cfmt = "{{:{}s}}".format(max([len(str(label)) for label in labels]))
+        max_len = max(len(str(label)) for label in labels)
+        cfmt = f"{{:{max_len}s}}"
         fmt.append(cfmt)
 
     if show_vals:
-        all_int = all(val == int(val) for val in vals)
-        if all_int:
-            cfmt = "{{:{}d}}".format(max([len(str(val)) for val in vals]))
+        if np.issubdtype(np.asarray(vals).dtype, np.integer):
+            max_len = max(len(str(val)) for val in vals)
+            cfmt = f"{{:{max_len}d}}"
         else:
             cfmt = "{}"
         fmt.append("[" + cfmt + "]")
@@ -54,7 +62,7 @@ def barh(
     return out
 
 
-def _get_matrix_of_eighths(counts, max_size, bar_width) -> List[List[int]]:
+def _get_matrix_of_eighths(counts, max_size: int, bar_width: int) -> List[List[int]]:
     """
     Returns a matrix of integers between 0-8 encoding bar lengths in histogram.
 
diff --git a/src/termplotlib/figure.py b/src/termplotlib/figure.py
index 73ef48f..731d578 100644
--- a/src/termplotlib/figure.py
+++ b/src/termplotlib/figure.py
@@ -1,3 +1,5 @@
+from typing import Optional
+
 from .barh import barh
 from .helpers import create_padding_tuple
 from .hist import hist
@@ -9,7 +11,7 @@ def figure(*args, **kwargs):
 
 
 class Figure:
-    def __init__(self, width=None, padding=0):
+    def __init__(self, width: Optional[int] = None, padding: int = 0):
         self._content = []
         self._width = width
         self._subfigures = None
diff --git a/src/termplotlib/helpers.py b/src/termplotlib/helpers.py
index 262ad3f..9b1df73 100644
--- a/src/termplotlib/helpers.py
+++ b/src/termplotlib/helpers.py
@@ -1,7 +1,8 @@
 import sys
+from typing import List, Tuple, Union
 
 
-def create_padding_tuple(padding):
+def create_padding_tuple(padding: Union[int, List[int], Tuple[int, int]]):
     # self._padding is a 4-tuple: top, right, bottom, left (just like CSS)
     if isinstance(padding, int):
         out = (padding, padding, padding, padding)
diff --git a/src/termplotlib/hist.py b/src/termplotlib/hist.py
index d72aa92..2393815 100644
--- a/src/termplotlib/hist.py
+++ b/src/termplotlib/hist.py
@@ -1,17 +1,18 @@
+from typing import List, Optional
+
 from .barh import _get_matrix_of_eighths, _trim_trailing_zeros, barh
 from .helpers import is_unicode_standard_output
 
 
 def hist(
-    counts,
-    bin_edges,
-    orientation="vertical",
-    max_width=40,
-    bins=20,
+    counts: List[int],
+    bin_edges: List[float],
+    orientation: str = "vertical",
+    max_width: int = 40,
     grid=None,
-    bar_width=1,
-    strip=False,
-    force_ascii=False,
+    bar_width: int = 1,
+    strip: bool = False,
+    force_ascii: bool = False,
 ):
     if orientation == "vertical":
         return hist_vertical(
@@ -26,26 +27,24 @@ def hist(
     return hist_horizontal(
         counts,
         bin_edges,
-        max_width=40,
-        bins=20,
+        max_width=max_width,
         bar_width=bar_width,
         force_ascii=force_ascii,
     )
 
 
 def hist_horizontal(
-    counts,
-    bin_edges,
-    max_width=40,
-    bins=20,
-    bar_width=1,
-    show_bin_edges=True,
-    show_counts=True,
-    force_ascii=False,
+    counts: List[int],
+    bin_edges: List[float],
+    max_width: int = 40,
+    bar_width: int = 1,
+    show_bin_edges: bool = True,
+    show_counts: bool = True,
+    force_ascii: bool = False,
 ):
     if show_bin_edges:
         labels = [
-            "{:+.2e} - {:+.2e}".format(bin_edges[k], bin_edges[k + 1])
+            f"{bin_edges[k]:+.2e} - {bin_edges[k+1]:+.2e}"
             for k in range(len(bin_edges) - 1)
         ]
     else:
@@ -59,24 +58,22 @@ def hist_horizontal(
         show_vals=show_counts,
         force_ascii=force_ascii,
     )
-
     return out
 
 
-def _flip(matrix):
+def _flip(matrix: List[List[int]]) -> List[List[int]]:
     """Mirrors a matrix left to right"""
     n_cols = len(matrix[0])
     return [[row[-(col_i + 1)] for row in matrix] for col_i in range(n_cols)]
 
 
 def hist_vertical(
-    counts,
-    bins=30,
-    max_height=10,
-    bar_width=2,
-    strip=False,
-    xgrid=None,
-    force_ascii=False,
+    counts: List[int],
+    max_height: int = 10,
+    bar_width: int = 2,
+    strip: bool = False,
+    xgrid: Optional[List[int]] = None,
+    force_ascii: bool = False,
 ):
     if xgrid is None:
         xgrid = []
diff --git a/src/termplotlib/plot.py b/src/termplotlib/plot.py
index 9704c6c..56768ca 100644
--- a/src/termplotlib/plot.py
+++ b/src/termplotlib/plot.py
@@ -1,19 +1,20 @@
 import subprocess
+from typing import List, Optional, Tuple
 
 
 def plot(
-    x,
-    y,
-    width=80,
-    height=25,
-    label=None,
-    xlim=None,
-    ylim=None,
-    xlabel=None,
-    title=None,
-    extra_gnuplot_arguments=None,
-    plot_command="plot '-' w lines",
-    ticks_scale=0,
+    x: List[float],
+    y: List[float],
+    width: int = 80,
+    height: int = 25,
+    label: Optional[str] = None,
+    xlim: Optional[Tuple[float, float]] = None,
+    ylim: Optional[Tuple[float, float]] = None,
+    xlabel: Optional[str] = None,
+    title: Optional[str] = None,
+    extra_gnuplot_arguments: Optional[List[str]] = None,
+    plot_command: str = "plot '-' w lines",
+    ticks_scale: int = 0,
 ):
     p = subprocess.Popen(
         ["gnuplot"],
@@ -29,10 +30,10 @@ def plot(
     gnuplot_input.append(f"set tics scale {ticks_scale}")
 
     if xlim:
-        gnuplot_input.append("set xrange [{}:{}]".format(xlim[0], xlim[1]))
+        gnuplot_input.append(f"set xrange [{xlim[0]}:{xlim[1]}]")
 
     if ylim:
-        gnuplot_input.append("set yrange [{}:{}]".format(ylim[0], ylim[1]))
+        gnuplot_input.append(f"set yrange [{ylim[0]}:{ylim[1]}]")
 
     if xlabel:
         gnuplot_input.append(f'set xlabel "{xlabel}"')
@@ -60,5 +61,5 @@ def plot(
     return _remove_empty_lines(out.decode())
 
 
-def _remove_empty_lines(string):
+def _remove_empty_lines(string: str):
     return string.split("\n")[1:-2]
diff --git a/src/termplotlib/subplot.py b/src/termplotlib/subplot.py
index 36125a0..812d78d 100644
--- a/src/termplotlib/subplot.py
+++ b/src/termplotlib/subplot.py
@@ -1,3 +1,5 @@
+from typing import List, Optional, Tuple, Union
+
 from .figure import Figure
 
 
@@ -9,10 +11,10 @@ class SubplotGrid:
     def __init__(
         self,
         layout,
-        width=None,
-        column_widths=None,
-        border_style="thin",
-        padding=(1, 2),
+        width: Optional[int] = None,
+        column_widths: Optional[List[int]] = None,
+        border_style: str = "thin",
+        padding: Union[int, List[int], Tuple[int, int]] = (1, 2),
     ):
         assert (
             len(layout) == 2
@@ -72,11 +74,9 @@ def get_string(self):
         ]
         column_widths = [
             max(
-                [
-                    len(line)
-                    for i in range(self._layout[0])
-                    for line in cstrings[i][j].split("\n")
-                ]
+                len(line)
+                for i in range(self._layout[0])
+                for line in cstrings[i][j].split("\n")
             )
             for j in range(self._layout[1])
         ]
diff --git a/tests/test_plot.py b/tests/test_plot.py
index 8584edc..0bad3d0 100644
--- a/tests/test_plot.py
+++ b/tests/test_plot.py
@@ -71,6 +71,8 @@ def test_plot_lim():
     )
     string = fig.get_string()
 
+    # for some reason, this gives a different result locally; perhaps a different
+    # gnuplotlib version
     ref = """                       header
 
     1 +---------------------------------------+