NHT changes plots combined fit (#166)

* NHT changes plots combined fit

* feat: Exception for illegal combination added and test fixed.

---------

Co-authored-by: Fabian Joswig <fabian.joswig@ed.ac.uk>

tests/fits_test.py

@@ -681,6 +681,42 @@ def test_combined_fit_no_autograd():
 
     pe.least_squares(xs, ys, funcs, num_grad=True)
 
+def test_plot_combined_fit_function():
+
+    def func_exp1(x):
+        return 0.3*anp.exp(0.5*x)
+
+    def func_exp2(x):
+        return 0.3*anp.exp(0.8*x)
+
+    xvals_b = np.arange(0,6)
+    xvals_a = np.arange(0,8)
+
+    def func_a(a,x):
+        return a[0]*anp.exp(a[1]*x)
+
+    def func_b(a,x):
+        return a[0]*anp.exp(a[2]*x)
+
+    corr_a = pe.Corr([pe.Obs([np.random.normal(item, item*1.5, 1000)],['ensemble1']) for item in func_exp1(xvals_a)])
+    corr_b = pe.Corr([pe.Obs([np.random.normal(item, item*1.4, 1000)],['ensemble1']) for item in func_exp2(xvals_b)])
+
+    funcs = {'a':func_a, 'b':func_b}
+    xs = {'a':xvals_a, 'b':xvals_b}
+    ys = {'a': [o[0] for o in corr_a.content],
+          'b': [o[0] for o in corr_b.content]}
+
+    corr_a.gm()
+    corr_b.gm()
+
+    comb_fit = pe.least_squares(xs, ys, funcs)
+
+    with pytest.raises(ValueError):
+        corr_a.show(x_range=[xs["a"][0], xs["a"][-1]], fit_res=comb_fit)
+
+    corr_a.show(x_range=[xs["a"][0], xs["a"][-1]], fit_res=comb_fit, fit_key="a")
+    corr_b.show(x_range=[xs["b"][0], xs["b"][-1]], fit_res=comb_fit, fit_key="b")
+
 
 def test_combined_fit_invalid_fit_functions():
     def func1(a, x):