An example for loss_elem2den func in lib_tw added.

README.md

@@ -46,6 +46,45 @@ lat.get_tw(file_name_lat_new)
 #    if lat.lst[i].typ=='STEERER'      : lat.lst[i].Bx =0.0; lat.lst[i].By =0.0
 ```
 
+lib_tw includes a function to convert loss/elem to loss/elem-length. An example:
+
+```
+# An example to make a bar plot with an output file of this script can be found in
+# https://gitlab01.esss.lu.se/ess-bp/pgfplots-examples/tree/master/loss.den
+
+from ess import lib_tw
+
+file_name_den ='density_file_name'
+file_name_ptrn='partran_out_file_name'
+
+file_name_dt  ='len.dt_dtl.v85.txt'  # File with (half cell length) vs (drift tube length)
+
+#-- An example for a density file
+
+den=DENSITY(file_name_den)
+s_from_den   =den.s
+loss_from_den=den.loss_pow         # For a file w/ single run
+#loss_from_den=den.loss_pow[Nrun]  # For a file w/ multi runs (e.g., file from an error study)
+
+loss_den_from_den=loss_elem2den(s_from_den,loss_from_den,file_name_dt,dtl_dt=5e-6)
+
+# Note:
+# - Be careful with the number of runs.
+# - The instances for losses are "loss_pow", "loss_num", "loss_pow_ave", and etc for DENSITY class.
+# - If file_name_dt isn't given, the half-cell lengths are used in stead of drift-tube lnegths.
+# - dlt_dt is a small number used to identify DTL_CEL elements.
+
+#-- An example for a partran out file
+
+ptrn=PARTRAN(file_name_ptrn)
+
+loss_den_from_ptrn=ptrn.loss_den(file_name_dt,dtl_dt=5e-6)
+
+# Note:
+# - For PARTRAN class, loss_elem2den function is already implemented as a method. 
+
+```
+
 ### Library
 
 To be done..