Runing on PyMC v5.3.0

Runing on arviz v0.15.1

Hi,

Hopefully somebody can see how to solve what seems like a simple problem of coords and ppc,s. which is to use az.plot_ppc(idata,coords) to plot ppcs for separate groups.

The problem appears solved in this example from : arviz.plot_ppc — ArviZ 0.16.1 documentation for getting az.plot_ppc() on indidual groups.

```
idata_radon = az.load_arviz_data('radon')
az.plot_ppc(idata_radon, data_pairs={"y":"y"},num_pp_samples=20)
obs_county = idata_radon.posterior["County"][df1.constant_data["county_idx"]]
idata_radon = idata_radon.assign_coords(obs_id=obs_county, groups="observed_vars")
az.plot_ppc(idata_radon, coords={'obs_id': ['ANOKA', 'BELTRAMI']}, flatten=[],num_pp_samples=20)
```

this 5 lines of code work perfectly for the radon idata.

The example I am working on is from Coordinates in PyMC & InferenceData Objects – Christian Luhmann – Python, math, etc. (cluhmann.github.io) which is explaining the use of coords in pymc.

This is the adapted code:

```
# CREAT SOME KNOWN BLOOD_PRESSURE DATA (BP) FOR TWO GROUPS (MALES AND FEMALES)
rng = np.random.default_rng(101010)
group_size = 100
systolic_f = np.random.normal(loc=123, scale=5, size=group_size)
systolic_f =rng.normal(loc=120, scale=2, size=group_size)#123
systolic_m = rng.normal(loc=150, scale=2, size=group_size)#127
systolic = np.hstack((systolic_f, systolic_m))
gender = (['female'] * group_size) + (['male'] * group_size)
df = pd.DataFrame({'bp':systolic, 'gender':gender})
df.reset_index()
df.head()
cat=pd.Categorical(df['gender'])
with pm.Model(coords = {'gender':['female','male']}) as labeled_model:
mu = pm.Normal('mu', 140, sigma=15, dims='gender')
likelihood = pm.Normal('likelihood',
mu=mu[cat.codes],
sigma=5,
observed=df['bp'])
idata_coords=pm.sample(chains=2)
idata_coords.extend(pm.sample_posterior_predictive(idata_coords))
az.plot_ppc(idata_coords,num_pp_samples=20)
```

this line works but to get the ppc plot for the ‘male’ and ‘female’ groups separately does not work!

```
az.plot_ppc(idata_coords,coords={'gender':['female','male']})
```

This last line of code does not work. Could anyone tell me how to write the last line of code using az.plot_ppc() and the coords to get plot predictions for the individual groups as appears to have been achieved in the Radon example above. Thank you in advance for examining code.