docs: add comment and documentation

R/MsBackend.R

@@ -561,7 +561,11 @@
 #' `data`. The second parameter `peaksVariables` allows to define which columns
 #' in `data` contain *peak variables* such as the m/z and intensity values of
 #' individual peaks per spectrum. The default for this parameter is
-#' `peaksVariables = c("mz", "intensity")`.
+#' `peaksVariables = c("mz", "intensity")`. Note that it is not supported to
+#' provide either `"mz"` or `"intensity"`, if provided, both need to be
+#' present in the data frame. Alternatively, the function also supports a data
+#' frame without m/z and intensity values, in which case a `Spectra` without
+#' mass peaks is created.
 #'
 #' Suggested columns of this `DataFrame` are:
 #'

R/MsBackendDataFrame.R

@@ -62,6 +62,8 @@ setMethod("backendInitialize", signature = "MsBackendDataFrame",
               if (!is(data, "DataFrame"))
                   stop("'data' has to be a 'DataFrame'")
               peaksVariables <- intersect(peaksVariables, colnames(data))
+              ## support either no m/z or intensity values provided, or both
+              ## have to be specified.
               if (sum(c("mz", "intensity") %in% peaksVariables) == 1L)
                   stop("Both \"mz\" and \"intensity\" peak variables ",
                        "need to be provided.")