How to generate histograms of multiple fields?

I have a large shapefile and I’m trying to automate a process to generate histograms of several fields with the MakeGraph tool, but I’m receiving the following error.

class ‘arcgisscripting.ExecuteError’>: Failed to execute. Parameters
are not valid. ERROR 001020: Input series error. Incompatible
parameter list. Click ‘Always reset parameters…’ check-box to reset
parameters. Failed to execute (MakeGraph).

Any idea why this is happening? My inputs are as follows…
input table = shapefile, fieldList is generated by input, workspace = output folder, input_template = a .grf or .tee (tried both), run is just a boolean true or false

Here’s my code:

# Import arcpy module
import arcpy

# Check out any necessary licenses
arcpy.CheckOutExtension("spatial")

input_table = arcpy.GetParameterAsText(0)

fieldList = arcpy.GetParameterAsText(1)

workspace = arcpy.GetParameterAsText(2)
arcpy.env.workspace = workspace

input_template = arcpy.GetParameterAsText(3)

run = arcpy.GetParameterAsText(4)

countBin = 10

# Boolean selection
if run == 'true':

    # Iterate over selected field names
    for fieldName in fieldList.split(';'):

        out_graph_name = "histogram"  
        out_graph_jpg = fieldName + "_histogram.jpg" 

        # Create the graph  
        graph = arcpy.Graph()  

        # Add a histogram series to the graph  
        graph.addSeriesHistogram(input_table, fieldName, countBin)  

        # Specify the title of the left axis  
        graph.graphAxis[0].title = "Count" 

        # Specify the title of the bottom axis  
        graph.graphAxis[2].title = fieldName  

        # Specify the title of the Graph  
        graph.graphPropsGeneral.title = fieldName + " Histogram" 

        # Output a graph, which is created in-memory  
        arcpy.MakeGraph_management(input_template, graph, out_graph_name)  

        # Save the graph as an image  
        arcpy.SaveGraph_management(out_graph_name, out_graph_jpg, "MAINTAIN_ASPECT_RATIO", 600, 375)

else:
    print "done"