diff --git a/common.py b/common.py
index 846c1b2..60acb8e 100644
--- a/common.py
+++ b/common.py
@@ -12,6 +12,11 @@ import sys
 import re
 
 
+# Increase major number for general changes, middle number for smaller changes
+# that can cause incompatibilities and minor number for regular fixes
+SARGRAPH_VERSION = "2.0.0"
+
+
 # Print an error message and exit with non-zero status
 def fail(msg):
     print(f"Error: {msg}", file=sys.stderr)
@@ -47,3 +52,16 @@ def scan(regex, conv, string):
     except ValueError:
         return None
     return value
+
+
+# Return True iff version string `a` is greater than or equal to `b`
+def is_version_ge(a, b):
+    a = [int(n) for n in a.split('.')]
+    b = [int(n) for n in b.split('.')]
+
+    if len(a) != len(b):
+        return len(a) > len(b)
+    for i, _ in enumerate(a):
+        if a[i] != b[i]:
+            break
+    return a[i] >= b[i]
diff --git a/graph.py b/graph.py
index 6ecd49c..171a84c 100755
--- a/graph.py
+++ b/graph.py
@@ -17,7 +17,7 @@ from common import *
 
 global gnuplot
 
-GNUPLOT_VERSION_EXPECTED = 5.0
+GNUPLOT_VERSION_EXPECTED = "5.0"
 
 START_DATE = ""
 END_DATE = ""
@@ -47,9 +47,9 @@ labels = []
 
 # Check if the avaliable gnuplot has a required version
 p = run_or_fail("gnuplot", "--version", stdout=subprocess.PIPE)
-version = scan(r"gnuplot (\S+)", float, p.stdout.readline().decode())
-if version < GNUPLOT_VERSION_EXPECTED:
-    fail(f"gnuplot version too low. Need at least {GNUPLOT_VERSION_EXPECTED} found {version[0]}")
+version = scan(r"gnuplot (\S+)", str, p.stdout.readline().decode())
+if not is_version_ge(version, GNUPLOT_VERSION_EXPECTED):
+    fail(f"gnuplot version too low. Need at least {GNUPLOT_VERSION_EXPECTED} found {version}")
 
 
 # Run a command in a running gnuplot process
@@ -94,6 +94,8 @@ def read_comments(session):
     global CPU_NAME
     global DURATION
 
+    data_version = None
+
     with open(f"{session}.txt", "r") as f:
         for line in f:
             value = None
@@ -114,6 +116,10 @@ def read_comments(session):
                 # Comments are not mixed with anything else, so skip
                 continue
 
+            value = scan("sargraph version: (\d+\.\d+)", str, line)
+            if value is not None:
+                data_version = value
+
             value = scan("machine: ([^,]+)", str, line)
             if value is not None:
                 UNAME = value
@@ -154,6 +160,9 @@ def read_comments(session):
             if value is not None:
                 AVERAGE_LOAD = value
 
+    if data_version != SARGRAPH_VERSION:
+        print("Warning: the data comes from an incompatible version of sargraph")
+
 
 def graph(session, fname='plot.png'):
     global OUTPUT_TYPE
@@ -193,7 +202,7 @@ def graph(session, fname='plot.png'):
 
     seconds_between = (edt - sdt).total_seconds()
     if seconds_between < 100:
-     seconds_between = 100
+        seconds_between = 100
 
     nsdt = sdt - datetime.timedelta(seconds = (seconds_between * 0.01))
     nedt = edt + datetime.timedelta(seconds = (seconds_between * 0.01))
diff --git a/watch.py b/watch.py
index fa4cca3..c377491 100755
--- a/watch.py
+++ b/watch.py
@@ -88,7 +88,8 @@ def initialize(session, machine):
                 break
 
     with open(f"{session}.txt", "w") as f:
-        print(f"# pid: {os.getpid()}",
+        print(f"# sargraph version: {SARGRAPH_VERSION}",
+              f"pid: {os.getpid()}",
               f"machine: {uname}",
               f"cpu count: {cpus}",
               f"cpu: {cpu_name}",