After fix

app.py

@@ -1,4 +1,4 @@
-from flask import Flask, render_template, request, redirect, url_for, send_file
+from flask import Flask, render_template, request, send_file
 import pandas as pd
 import matplotlib.pyplot as plt
 import numpy as np
@@ -6,14 +6,13 @@ import io
 import os
 
 app = Flask(__name__)
-app.config["UPLOAD_FOLDER"] = "uploads"
-os.makedirs(app.config["UPLOAD_FOLDER"], exist_ok=True)
 
-# Global variables to hold data between requests
-df1, df2, limits, cols_to_plot = None, None, {}, []
+# Store data globally (for demo simplicity — in production, use session or caching)
+data_cache = {"df1": None, "df2": None, "limits": {}, "cols": []}
+
 
 def process_files(golden_file, test_file):
-    global df1, df2, limits, cols_to_plot
+    """Load Excel files and prepare dataframes and limits."""
     limits_df1 = pd.read_excel(golden_file, nrows=4)
     df1 = pd.read_excel(golden_file)
     df1 = df1.drop([0, 1, 2, 3])
@@ -32,11 +31,18 @@ def process_files(golden_file, test_file):
         col: {"LL": limits_df1.iloc[0][col], "UL": limits_df1.iloc[1][col]}
         for col in limits_df1.columns
     }
-    return df1, df2, limits, cols_to_plot
+
+    # Store in cache for later retrieval
+    data_cache["df1"], data_cache["df2"], data_cache["limits"], data_cache["cols"] = df1, df2, limits, cols_to_plot
 
 
 def generate_plot(col):
-    """Generate a matplotlib plot for a single column."""
+    """Generate and return a PNG plot for a given column."""
+    df1, df2, limits = data_cache["df1"], data_cache["df2"], data_cache["limits"]
+
+    if df1 is None or col not in df1.columns:
+        raise ValueError(f"Data not loaded or column '{col}' missing")
+
     plt.figure(figsize=(6, 4))
 
     x1 = np.arange(1, len(df1[col]) + 1)
@@ -69,8 +75,6 @@ def generate_plot(col):
 
 @app.route("/", methods=["GET", "POST"])
 def index():
-    global df1, df2, limits, cols_to_plot
-
     if request.method == "POST":
         golden_file = request.files.get("golden_file")
         test_file = request.files.get("test_file")
@@ -79,27 +83,31 @@ def index():
             return render_template("index.html", error="Please upload both files.")
 
         try:
-            df1, df2, limits, cols_to_plot = process_files(golden_file, test_file)
+            process_files(golden_file, test_file)
+
+            cols = data_cache["cols"]
+            preview_cols = cols[:3] if len(cols) >= 3 else cols
 
-            # Show the first 3 plots initially
-            preview_cols = cols_to_plot[:3]
             return render_template(
                 "plot.html",
-                cols=cols_to_plot,
-                preview_cols=preview_cols,
+                cols=cols,
+                preview_cols=preview_cols
             )
 
         except Exception as e:
-            return render_template("index.html", error=f"Error: {str(e)}")
+            return render_template("index.html", error=f"Error processing files: {str(e)}")
 
     return render_template("index.html")
 
 
 @app.route("/plot_image/<col>")
 def plot_image(col):
-    """Return plot as PNG stream for the selected column."""
-    buf = generate_plot(col)
-    return send_file(buf, mimetype="image/png")
+    """Return plot image stream for selected column."""
+    try:
+        buf = generate_plot(col)
+        return send_file(buf, mimetype="image/png")
+    except Exception as e:
+        return f"Error generating plot: {str(e)}"
 
 
 if __name__ == "__main__":

templates/plot.html

@@ -36,10 +36,14 @@
     <h2>📈 IPM Data Comparison</h2>
 
     <h3>Preview (First few plots):</h3>
-    {% for c in preview_cols %}
-        <div class="preview-title">{{ c }}</div>
-        <img src="/plot_image/{{ c }}" alt="{{ c }}">
-    {% endfor %}
+    {% if preview_cols %}
+        {% for c in preview_cols %}
+            <div class="preview-title">{{ c }}</div>
+            <img src="/plot_image/{{ c }}" alt="{{ c }}">
+        {% endfor %}
+    {% else %}
+        <p>No columns found for preview.</p>
+    {% endif %}
 
     <hr>
     <h3>View Other Columns</h3>