diff --git a/src/cristallina/__init__.py b/src/cristallina/__init__.py
index eace891..2f0b74a 100644
--- a/src/cristallina/__init__.py
+++ b/src/cristallina/__init__.py
@@ -15,6 +15,10 @@ except PackageNotFoundError:  # pragma: no cover
 finally:
     del version, PackageNotFoundError
 
+from . import config
 from . import utils
-from . import analysis
 from . import plot
+
+# analysis requires a pgroup for persistent caching, we try
+# heuristics but this can also be (re-)set after the import.
+from . import analysis
\ No newline at end of file
diff --git a/src/cristallina/analysis.py b/src/cristallina/analysis.py
index c736ab3..928096c 100644
--- a/src/cristallina/analysis.py
+++ b/src/cristallina/analysis.py
@@ -13,23 +13,40 @@ from joblib import Parallel, delayed, Memory
 from . import utils
 from .utils import ROI
 
-# Ideally we automatically determine the cache directory tied to the p-group
-# If this fails we try with a fixed path for now, and finally use the 
-# current temporary directory.
-try:
-    pgroup = utils.heuristic_extract_pgroup()
-    location = f"/das/work/units/cristallina/p{pgroup}/cachedir"
-except KeyError as e:
-    print(e)
-    location = "/das/work/units/cristallina/p19739/cachedir"
+memory = None
 
-try:
-    memory = Memory(location, verbose=0, compress=2)
-except PermissionError as e:
-    location = "/tmp"
-    memory = Memory(location, verbose=0, compress=2)
+def setup_cachedirs(pgroup=None, cachedir=None):
+    """ 
+    Sets the path to a persistent cache directory either from the given p-group (e.g. "p20841")
+    or an explicitly given directory.
+
+    If heuristics fail we use "/tmp" as a non-persistent alternative.
+    """
     
+    global memory
+    if cachedir is not None:
+        # explicit directory given, use this choice
+        memory = Memory(cachedir, verbose=0, compress=2)
+        return 
 
+    try:
+        if pgroup is None:
+            pgroup_no = utils.heuristic_extract_pgroup()
+        else:
+            parts = re.split(r'(\d.*)', pgroup)      # ['p', '2343', '']
+            pgroup_no = parts[-2]
+        cachedir = f"/das/work/units/cristallina/p{pgroup_no}/cachedir"
+    except KeyError as e:
+        print(e)
+        cachedir = "/das/work/units/cristallina/p19739/cachedir"
+
+    try:
+        memory = Memory(cachedir, verbose=0, compress=2)
+    except PermissionError as e:
+        location = "/tmp"
+        memory = Memory(cachedir, verbose=0, compress=2)
+    
+setup_cachedirs()
 
 @memory.cache(ignore=["batch_size"])  # we ignore batch_size for caching purposes
 def sum_images(
@@ -42,11 +59,16 @@ def sum_images(
 ):
     """
     Sums a given region of interest (roi) for an image channel from a
-    given fileset (e.g. "run0352/data/acq0001.*.h5").
+    given fileset (e.g. "run0352/data/acq0001.*.h5" or step.fnames from a SFScanInfo object).
 
     Allows alignment, i.e. reducing only to a common subset with other channels.
 
-    Summation is performed in batches, preview only sums and returns the first batch.
+    Summation is performed in batches to reduce maximum memory requirements.
+    
+    Preview only sums and returns the first batch.
+
+    Returns a dictionary ({"JF16T03V01_intensity":[11, 18, 21, 55, ...]}) 
+    with the given channel intensity for each pulse and corresponding pulse id.
     """
 
     with SFDataFiles(*fileset) as data:
diff --git a/src/cristallina/config.py b/src/cristallina/config.py
new file mode 100644
index 0000000..a4f7357
--- /dev/null
+++ b/src/cristallina/config.py
@@ -0,0 +1,3 @@
+
+# not used yet
+# PGROUP = ''
\ No newline at end of file
diff --git a/src/cristallina/plot.py b/src/cristallina/plot.py
index c7e11e9..e4fca2c 100644
--- a/src/cristallina/plot.py
+++ b/src/cristallina/plot.py
@@ -73,7 +73,12 @@ def plot_correlation(x, y, ax=None, **ax_kwargs):
 
     return ax, r
 
-def plot_channel(data, channel_name, ax=None):
+def plot_channel(data : SFDataFiles, channel_name, ax=None):
+    """ 
+    Plots a given channel from an SFDataFiles object. 
+    
+    Optionally: a matplotlib axis to plot into
+    """
 
     channel_dim = len(data[channel_name].shape)
     # dim == 3: a 2D Image
@@ -105,7 +110,7 @@ def axis_styling(ax, channel_name, description):
     )
 
 
-def plot_1d_channel(data, channel_name, ax=None):
+def plot_1d_channel(data : SFDataFiles, channel_name, ax=None):
     """
     Plots channel data for a channel that contains a single numeric value per pulse.
     """
@@ -126,7 +131,7 @@ def plot_1d_channel(data, channel_name, ax=None):
     axis_styling(ax, channel_name, description)
 
 
-def plot_2d_channel(data, channel_name, ax=None):
+def plot_2d_channel(data : SFDataFiles, channel_name, ax=None):
     """
     Plots channel data for a channel that contains a 1d array of numeric values per pulse.
     """
@@ -148,7 +153,7 @@ def plot_2d_channel(data, channel_name, ax=None):
     axis_styling(ax, channel_name, description)
 
 
-def plot_image_channel(data, channel_name, pulse=0, ax=None, rois=None, norms=None):
+def plot_image_channel(data : SFDataFiles, channel_name, pulse=0, ax=None, rois=None, norms=None):
     """
     Plots channel data for a channel that contains an image (2d array) of numeric values per pulse.
     """
@@ -186,7 +191,7 @@ def plot_image_channel(data, channel_name, pulse=0, ax=None, rois=None, norms=No
     axis_styling(ax, channel_name, description)
     plt.legend(loc=4)    
 
-def plot_spectrum_channel(data, channel_name_x, channel_name_y, average=True, pulse=0, ax=None):
+def plot_spectrum_channel(data : SFDataFiles, channel_name_x, channel_name_y, average=True, pulse=0, ax=None):
     """
     Plots channel data for two channels where the first is taken as the (constant) x-axis
     and the second as the y-axis (here we take by default the mean over the individual pulses).
diff --git a/src/cristallina/utils.py b/src/cristallina/utils.py
index 018913c..bdcbb07 100644
--- a/src/cristallina/utils.py
+++ b/src/cristallina/utils.py
@@ -136,7 +136,9 @@ def process_run(run_number, rois,detector='JF16T03V01', calculate =None, only_sh
 class ROI:
     """Definition of region of interest (ROI) in image coordinates.
 
-    Example: ROI(left=10, right=20, bottom=100, top=200)
+    Example: ROI(left=10, right=20, bottom=100, top=200). 
+
+    Directions assume that lower left corner of image is at (x=0, y=0).
     """
 
     def __init__(
@@ -202,7 +204,7 @@ class ROI:
         return f"ROI(bottom={self.bottom},top={self.top},left={self.left},right={self.right})"
 
     def __eq__(self, other):
-        # we disregard the name
+        # we disregard the name for comparisons
         return (self.left, self.right, self.bottom, self.top) == (other.left, other.right, other.bottom, other.top) 
     
     def __ne__(self, other):