Allow dimensions in NXdata to be lists of strings

base_classes/NXdata.nxdl.xml

@@ -115,6 +115,51 @@
 			the axes attribute can be omitted.
 		</doc>
 	</attribute>
+	<attribute name="default_slice" type="NX_CHAR_OR_NUMBER">
+		<doc>
+			Which slice of data to show in a plot by default. This is useful especially for
+			datasets with more than 2 dimensions.
+
+			Should be an array of length equal to the number of dimensions
+			in the data, with the following possible values:
+
+			* ".": All the data in this dimension should be included
+			* Integer: Only this slice should be used.
+			* String: Only this slice should be used. Use if ``AXISNAME`` is a string
+			  array.
+
+			Example::
+
+			  data:NXdata
+				@signal = "data"
+				@axes = ["image_id", "channel", ".", "."]
+				@image_id_indices = 0
+				@channel_indices = 1
+				@default_slice = [".", "difference", ".", "."]
+				image_id = [1, ..., nP]
+				channel = ["threshold_1", "threshold_2", "difference"]
+				data = uint[nP, nC, i, j]
+
+			Here, a data array with four dimensions, including the number of images
+			(nP) and number of channels (nC), specifies more dimensions than can be
+			visualized with a 2D image viewer for a given image.  Therefore the
+			default_slice attribute specifies that the "difference" channel should be
+			shown by default.
+
+			Alternate version using an integer would look like this (note 2 is a string)::
+
+			  data:NXdata
+				@signal = "data"
+				@axes = ["image_id", "channel", ".", "."]
+				@image_id_indices = 0
+				@channel_indices = 1
+				@default_slice = [".", "2", ".", "."]
+				image_id = [1, ..., nP]
+				channel = ["threshold_1", "threshold_2", "difference"]
+				data = uint[nP, nC, i, j]
+
+		</doc>
+	</attribute>
 	<attribute name="AXISNAME_indices" type="NX_INT">
 		<!-- 
 			nxdl.xsd rules do not allow us to show this as a variable name 
@@ -273,14 +318,18 @@
 		but not to describe how the data is to be plotted.
 		(https://www.nexusformat.org/NIAC2018Minutes.html#nxdata-plottype--attribute)
 	</doc>
-	<field name="AXISNAME" type="NX_NUMBER" nameType="any">
+	<field name="AXISNAME" type="NX_CHAR_OR_NUMBER" nameType="any">
 		<doc>
-	 		Dimension scale defining an axis of the data.
-	 		Client is responsible for defining the dimensions of the data.
-	 		The name of this field may be changed to fit the circumstances.
-	 		Standard NeXus client tools will use the attributes to determine
-	 		how to use this field.
-	 	</doc>
+			Dimension scale defining an axis of the data.
+			Client is responsible for defining the dimensions of the data.
+			The name of this field may be changed to fit the circumstances.
+			Standard NeXus client tools will use the attributes to determine
+			how to use this field.
+
+			Most dimensions scales will be sequences of numbers but if an
+			axis is better represented using names, such as channel names,
+			an array of NX_CHAR can be provided.
+		</doc>
 		<dimensions rank="1">
 			<doc>
 				A *dimension scale* must have a rank of 1 and has length ``n``.

nxdlTypes.xsd

@@ -466,6 +466,7 @@
 			nxdl:NX_POSINT
 			nxdl:NX_QUATERNION
 			nxdl:NX_UINT
+			nxdl:NX_CHAR_OR_NUMBER
 			"/>
 	</xs:simpleType>
 
@@ -619,4 +620,14 @@
 		<xs:restriction base="xs:unsignedByte" />
 	</xs:simpleType>
 
+	<xs:simpleType name="NX_CHAR_OR_NUMBER">
+		<xs:annotation>
+			<xs:documentation>Any valid character string or NeXus number representation</xs:documentation>
+		</xs:annotation>
+		<xs:union memberTypes="
+			nxdl:NX_NUMBER
+			nxdl:NX_CHAR
+			" />
+	</xs:simpleType>
+
 </xs:schema>