Clearing mt tkinter canvas

from numpy import *
from scipy.optimize import curve_fit
import matplotlib.pyplot as plt
from Tkinter import *
import tkMessageBox
from pylab import savefig
from matplotlib.backends.backend_tkagg import FigureCanvasTkAgg, NavigationToolbar2TkAgg
import matplotlib.backend_bases       
from matplotlib.figure import Figure
c = open('napalm.dat','r')
j=c.read()

def rutherford():
	tkMessageBox.showinfo("Rutherford Scattering", j)
def N():
	tkMessageBox.showinfo("Variable N", "Number of alpha particles incident on foil")

def T():
	tkMessageBox.showinfo('Variable t', 'Thickness of foil in metres')

def K():
	tkMessageBox.showinfo('Variable E', 'Kinetic energy of alpha particles in joules')
	
def atom():
	tkMessageBox.showinfo('Variable Z', 'Atomic number of element from which the foil is made, or the name of the element')
	
def exp():
	import matplotlib.pyplot as plt
	k=genfromtxt('experiment.dat')
	print k
	plt.plot(k)
	plt.show()
	
class App:
	def __init__(self, master):
		frame = Frame(master) #creates the first frame
		
		Button(frame, text = "?", command=N).grid(row=1,column=2) #various help buttons
		Button(frame, text = "?", command=T).grid(row=2,column=2)
		Button(frame, text = "?", command=K).grid(row=3,column=2)
		Button(frame, text = "?", command=atom).grid(row=4,column=2)
		Button(frame, text='Plot from experimental data',command=exp).grid(row=5,column=5)
				
		Nlabel = Label(frame, text = "N =").grid(row=1, column=0, sticky=E) #labels next to the entry frame
		tlabel = Label(frame, text="t =").grid(row=2,column=0, sticky=E)	
		Elabel = Label(frame, text="E =").grid(row=3,column=0, sticky=E)		
		Zlabel = Label(frame, text="Z =").grid(row=4, column=0, sticky=E)
		
		eN=Entry(frame)
		eN.grid(row=1,column=1)
		et=Entry(frame)
		et.grid(row=2,column=1)
		eE=Entry(frame)
		eE.grid(row=3,column=1)
		eZ=Entry(frame)
		eZ.grid(row=4,column=1)
				
		def flup():				#the plot
			N=int(eN.get())		#turn string into integer
			t=float(et.get())		#turn string into float
			E=float(eE.get())
			Z=float(eZ.get())
			r=10*(10**-10)
			e=1.602*(10**-19)
			a=5*(10**30)
			i = linspace(math.pi/80, math.pi, 1000)
			list =[]
			
			for p in i:
				b = (N*a*t)/(16.0*(r**2))
				c = ((2.0*Z*(e**2))/(E*4.0*math.pi*8.854*(10**-12)))**2
				d = (1.0/(math.sin(p/2.0)))**4
				n=b*c*d
				list.append(n)
			
			f=Figure(figsize=(5,4), dpi=100)
			g=f.add_subplot(111)
			g.plot(i,list)
			
			
			canvas=FigureCanvasTkAgg(f, master=master)     #I think the problem is somewhere around here
			h=canvas.get_tk_widget()				
			h.pack(side=LEFT, fill=BOTH, expand=1)
			
			
		
			def clear():
				canvas.get_tk_widget().delete("all")
			
			
			
			delete=Button(frame,text='Clear All', command=clear)
			delete.grid(row=5,column=2)
			
			#update=Button(frame, text = "Update", command=flup) #this button makes the plot
			#update.grid(row=5,column=3)
		
		bgra=Button(frame, text = "Plot", command=flup) #this button makes the plot
		bgra.grid(row=5,column=1)
		
		frame2=Frame(master)
		
		b1=Button(frame2, text="Rutherford Scattering??", command=rutherford)
		b1.pack(side=LEFT)
		frame2.pack()
		frame.pack()
		
		
	
root = Tk()
app = App(root)
root.mainloop()

		
			def clear():
				canvas.get_tk_widget().destroy()
				#canvas.get_tk_widget().delete("all")