diff --git a/pydatview/common.py b/pydatview/common.py
index 1002c5e..7d15bc6 100644
--- a/pydatview/common.py
+++ b/pydatview/common.py
@@ -511,8 +511,15 @@ def exception2string(excp, iMax=40, prefix='    | ', prevStack=True):
 # ---  
 # --------------------------------------------------------------------------------{
 def isString(x):
-    b = x.dtype == object and isinstance(x.values[0], str)
-    return b 
+    if len(x) == 0:
+        return False
+    if isinstance(x, list):
+        return isinstance(x[0], str)
+    if hasattr(x, 'values'):  # handles pandas Series/DataFrame and xarray DataArray
+        val = x.values.flat[0]
+    else:  # handles numpy array
+        val = x.flat[0]
+    return isinstance(val, str)
 
 def isDate(x):
     return np.issubdtype(x.dtype, np.datetime64)
diff --git a/pydatview/plotdata.py b/pydatview/plotdata.py
index 820dd82..372625d 100644
--- a/pydatview/plotdata.py
+++ b/pydatview/plotdata.py
@@ -1,5 +1,6 @@
 import os
 import numpy as np
+import pandas
 from pydatview.common import no_unit, unit, inverse_unit, splitunit, has_chinese_char
 from pydatview.common import isString, isDate, getDt
 from pydatview.common import unique, pretty_num, pretty_time, pretty_date
@@ -10,6 +11,7 @@
 except AttributeError:
     trapz = np.trapz
 
+MAX_UNIQUE_STRING_TO_PLOT = 1000 # Potentially put this in user file
 
 # --------------------------------------------------------------------------------}
 # --- PlotDataList functions
@@ -70,6 +72,7 @@ def __init__(PD, x=None, y=None, sx='', sy=''):
         PD.xyMeasInput1 = (None, None)
         PD.xyMeasInput2 = (None, None)
         PD.xyMeas      = [(None,None)]*2 # 2 measures for now
+        PD.MAX_UNIQUE_STRING_TO_PLOT = MAX_UNIQUE_STRING_TO_PLOT
 
         if x is not None and y is not None:
             PD.fromXY(x,y,sx,sy)
@@ -118,12 +121,15 @@ def _post_init(PD, pipeline=None):
 
 
         # --- Store stats
-        n=len(PD.y)
-        if n>1000:
-            if (PD.xIsString):
-                raise Exception('Error: x values contain more than 1000 string. This is not suitable for plotting.\n\nPlease select another column for table: {}\nProblematic column: {}\n'.format(PD.st,PD.sx))
-            if (PD.yIsString):
-                raise Exception('Error: y values contain more than 1000 string. This is not suitable for plotting.\n\nPlease select another column for table: {}\nProblematic column: {}\n'.format(PD.st,PD.sy))
+        n  = len(PD.y)
+        if PD.xIsString:
+            nu = len(np.unique(PD.x))
+            if nu > PD.MAX_UNIQUE_STRING_TO_PLOT:
+                raise Exception(f'Error: x values contain more than {PD.MAX_UNIQUE_STRING_TO_PLOT} unique strings. This is not suitable for plotting.\n\nPlease select another column for table: {PD.st}\nProblematic column: {PD.sx}\n')
+        if PD.yIsString:
+            nu = len(np.unique(PD.y))
+            if nu > PD.MAX_UNIQUE_STRING_TO_PLOT:
+                raise Exception(f'Error: y values contain more than {PD.MAX_UNIQUE_STRING_TO_PLOT} unique strings. This is not suitable for plotting.\n\nPlease select another column for table: {PD.st}\nProblematic column: {PD.sy}\n')
 
         PD.needChineseFont = has_chinese_char(PD.sy) or has_chinese_char(PD.sx)
         # Stats of the raw data (computed once and for all, since it can be expensive for large dataset
diff --git a/tests/test_plotdata.py b/tests/test_plotdata.py
index 6575145..28f47f9 100644
--- a/tests/test_plotdata.py
+++ b/tests/test_plotdata.py
@@ -5,7 +5,7 @@
 import matplotlib.pyplot as plt
 
 
-from pydatview.plotdata import PlotData
+from pydatview.plotdata import PlotData, MAX_UNIQUE_STRING_TO_PLOT
 
 class TestPlotData(unittest.TestCase):
 
@@ -115,7 +115,21 @@ def test_fatigue(self):
         np.testing.assert_almost_equal(v, 9.4714702, 3)
 
 
+    def test_plotManyStrings(self):
+        # Test for an array of size less than MAX_UNIQUE_STRING_TO_PLOT
+        x = np.linspace(-2, 2, MAX_UNIQUE_STRING_TO_PLOT)
+        y = np.asarray([f"s_{i}" for i in range(1, len(x) + 1)])
+        PD = PlotData(x,y)
+        self.assertEqual(PD.xIsString, False)
+        self.assertEqual(PD.yIsString, True)
+        # Test for a larger array
+        x = np.linspace(-2, 2, MAX_UNIQUE_STRING_TO_PLOT+1)
+        y = np.asarray([f"s_{i}" for i in range(1, len(x) + 1)])
+        with self.assertRaises(Exception):
+            PD = PlotData(x,y)
+
 
 
 if __name__ == '__main__':
-    unittest.main()
+    TestPlotData().test_plotManyStrings()
+#     unittest.main()